iPhone 4 on left, iPhone 3.x on right
iLounge has posted an excellent summary of changes in iPhone 4 on an app-by-app basis. They do a good job covering much of what has been discussed including multitasking, wallpapers, folders, mail and more.
iLounge, however, also noted that in iPhone 4's Safari application, Apple has removed the "Google" branding from the search button and simply replaced it with "Search". While seemingly a minor detail, Apple has been said to be in discussions with Microsoft to replace Google as the default search engine on the iPhone. If this were to happen, Google would likely still be a user enable-able option.
Note, the iPad's Safari also uses the "Search" button on the keyboard. "Google" still appears in the search window itself in Safari.