For people using POP access in their iPhone, the new update for the device will automatically convert your email to IMAP without you knowing. Why does that matter? Well, if you are in the habit of deleting messages from your iPhone after you've read them, those messages will now be sent to your trash in Gmail too.
It's unfortunate that most users will be updated and probably never know what just hit them, but for those who are in the know, it's actually a pretty good feature. You can think of IMAP as simply a different interface for your actual Gmail account -- anything you do here will be done right in Gmail too. That means that actions like moving, deleting, and replying are instantly broadcast to any client using IMAP.
Attention iPhone POP users
If you configured Gmail access on your iPhone by tapping the large 'Gmail' icon in mail setup, please note that installing software update 1.1.3 will automatically convert Gmail on your iPhone from POP access to IMAP access. This means that actions you take on your iPhone will now sync with actions in your Gmail web interface (i.e. messages you read on your iPhone will show up as 'Read' in the web interface). This also means that messages you delete on your iPhone will be moved to the Trash folder in the Gmail web interface, and thus deleted after 30 days. Please exercise caution when deleting messages, as this behavior is markedly different from POP access.