The are lots of great new features in iOS 4, hundreds Apple claims. Here are 12 cool things you can do with iPhone iOS 4.
Read iBooks on your iPhone
The iBooks app is now available for iPhone. Purchase ebooks from the iBooks Store and read them anywhere you go. iBooks even syncs bookmarks and the last page read. So, if you read books on an iPad and iPod touch, too, you can open an ebook and continue reading right from where you had left off.
View and Save PDFs in iBooks
You can view PDFs in iBooks, too. And keep them nicely organized in their own library shelves in iBooks. Here's how to get a PDF into iBooks. Email yourself a PDF file. Open the email and then press and hold your finger down on the attachment icon. A menu will pop-up. Tap "Open in iBooks."
Keep apps running - multitasking!
Multitasking in iOS 4 makes it possible to run more than one app at a time and switch between them fast. Lookup movie times while you compose an email. Keep the Camera app open while you take notes in Evernote. Leave a book open in iBooks while you search the web with Safari. Keep Twitter open, so you can tweet something fast, no matter what you're doing on your iPhone. You'll find lots of ways to take advantage of multitasking.
Double-click the home button. The screen will side up to reval icons of apps that are open. If there are more than four open, swipe left over the bottom of the screen to see them. Tap an app's icon to switch to it quickly. (Sorry, this feature is not available on iPhone 3G and earlier models.)
Listen to music with Pandora
The nice thing about the iPod app on the iPhone is that it continues to play music, even as you use another app. With iOS 4, the Pandora app can now do the same. So, enjoy discovering new music as use do other things on you phone.
Create playlists
Music playlist creating is now mobile. On your iPhone, launch the iPod app. Tap the Playlists icon (at the lower left of the screen). Then, tap "Add Playlist..." Enter the name for your new playlist. Tap songs to add them to the playlist. Tap the More button to find songs by genre, albums, etc. Playlists created this way are copied to your Mac the next time you sync. You can even edit playlists you created on your Mac or PC.
Organize you apps into Folders
Tidy-up your home screens by putting apps into folders. A folder for games. A folder for news app. A folder for web bookmarks. A folder for radio apps. The more apps you put into folders, the fewer home screens you'll have. 
Fewer home screens means less swiping to find an app. Organize apps in a way that makes sense to you, that let's you find apps easily.
Hold your finger down on an app icon in a home screen. The icons will begin to wiggle. Drag the icon onto another app icon. When you let go, the apps are placed into a folder. You can add more apps to a folder by dragging their icons onto the folder icon. Click the Home button to save.
View all your email in one Unified Inbox
iPhone makes it easy to read email from Google, MobileMe, and other email services. Now, in iOS 4, you can view all your email in one unified inbox that displays email from the inboxes of all your email accounts.
Type with a full-sized keyboard
Sure, iPhone does a lot to let you type well on it's little keyboard. Still, typing on a full-sized keyboard is so much better. iOS 4 let's you use a Bluetooth keyboard with your iPhone. (Sorry, this feature is not available on iPhone 3Gand earlier models.)
Organize and view photos by Albums, Places, Events and Faces
If you use iPhoto '09, you know that photos can be organized by albums and automatically by events, faces and places. Now, in iOS 4, the Photos app has this feature.
Choose a Lockscreen Wallpaper and a Home Screen Wallpaper
Now, you can use a different image for the Lock Screen (the screen with the "slide to unlock" bar) and Home Screens (the screens that display app icons). Open the Settings app and then tap Wallpaper. Tap the screen thumbnail images to select the wallpapers you want to use. (Sorry, this feature is not available on iPhone 3G and earlier models.)
Zoom-in for a photo.
5x Digital Zoom means you can get in closer to the action when you snap a photo with your iPhone. Slide the zoom control to frame your photo just right. Sure, it's not as good as optical zoom, but it works surprisingly well.
Search your phone, the web and Wikipedia, too.
No matter what you're doing on your iPhone, Spotlight search is never more than three quick clicks of the Home button away. And now, Spotlight let's you search the web and Wikipedia, too. When you're in an app, click the Home button to return to a Home screen. Click the Home button again to jump to the left-most Home screen. One more click takes you to Spotlight. Type what you're looking for into the search field. At the end of the search results, you'll see the option to use Google to search the web or search Wikipedia.
Rotation lock
The iPad has a switch that locks the orientation, so that no matter how you hold it, the screen won't rotate. iOS 4 has a similar feature, but it's a little hidden. Double- tap the Home button. (The screen will slide up and you'll see the apps that are "multitasking.") Now, swipe right across the bottom of the screen. You'll see iPod controls and, to the very left, the Portrait Orientation Lock button. Tap to lock the screen in portrait orientation.
You can use keyboard shortcuts for common editing tasks such as Undo (command key + 'z'), Copy (command key + 'c') and Paste (command key + 'v').
Turn on the Bluetooth keyboard. Then, on your iPad, open the Settings app. Tap General and then tap Bluetooth. Make sure Bluetooth is turned on. After a moment, you'll see your keyboard listed. Tap it.
bluetooth keyboard, 
