After lagging behind Yahoo!, Google has added IMAP support to Gmail. This means that if you want to read your mail using a client like Thunderbird, Apple’s Mail, or MobileMail on the iPhone, any messages that you read on the client will be marked as read on Gmail’s web interface. It was annoying previously that whenever I opened up my email on my iPhone, all the mail that I’d already read during the day would pop up as new. Here’s the announcement from Google, and here’s a list of instructions on how to set it up. The iPhone gets its own page, and a video showing you how to set it up. There’s also a page explaining how they’ve translated Gmail’s Labels to IMAP folders. I don’t use labels too much—my mail needs are pretty simple—but this is a big deal because Gmail lets you apply multiple labels to the same message, while most mail systems tend to only allow a message be in one folder.
You can check to see if your account has had IMAP access turned on yet by going to Settings → Forwarding and POP. If it has a section for IMAP access, you’re set. If not, you’ll have to wait until Gmail upgrades your account. It doesn’t seem that Google Apps accounts have been upgraded. Mine hasn’t at least.
Update
Seems Google Apps accounts are being upgraded, too, so I’ll just have to wait until mine gets switched on. Also, you can reportedly use this to upload old mail to Gmail, if you have any stuff sitting around from other accounts.
Update 2
Here’s the post on the Gmail blog.
Update 3
My account’s active now. Here are some tips on how to set up your Drafts, Sent, and Trash folders to sync back to Gmail from Mail and MobileMail.