Gmail is getting support for IMAP clients like Outlook, Thunderbird, and the iPhone. This means Gmail users will no longer be limited to the Gmail.com user interface or to the weak integration they might get from clients using the much more rudimentary POP e-mail protocol.
There's no word on the official rollout schedule for IMAP support. Some users have it, some don't. I do. Don't ask me why. To see if you have support, click the "Settings" link to see if you have a "Forwarding and POP/IMAP" tab.
Update: IMAP rollout should be complete within a few days, according to the Official Gmail blog.
If your account is IMAP enabled, these instructions will walk you through configuring your account and client software.
Why does this matter? Because IMAP is a two-way e-mail protocol. IMAP client applications can interact with IMAP servers, like Gmail's, so data on both sides stays in sync. If you have multiple IMAP client applications, they'll all show the same e-mail messages (and indicate which messages have been read) as long as they stay connected. However, Gmail handles e-mail differently from most standard e-mail client software, so the integration takes getting used to. For example, if you move a message from one folder to another in your desktop e-mail client (such as Outlook or Thunderbird), the message's label in Gmail will change to match--since Gmail doesn't have folders, per se. Google's IMAP behavior chart lists the differences.
Gmail's automatic message threading (where it groups messages with the same subject line) also doesn't translate to IMAP, although some client applications may offer their own threading features.
I think it's great to have rich client-side support for Web-based e-mail, and vice versa (example: Outlook Web Access). However, Gmail's native interface is so weird that no client I know of is a good match for it. It is very nice to be able to access more of Gmail's features from a real desktop client, but it requires a bit more of a mental context switch than I think most people will want. I tried using Gmail from Outlook, and I didn't like it. That's partly because Outlook's IMAP handling is quirky, but also because it's so different from Gmail's native interface. What would be more useful, I think, would be an offline Gmail interface using Google Gears.
Also, it's worth noting that the IMAP support is one-way: You can use IMAP clients to read and send your Gmail. You cannot use the Gmail interface to access e-mail from non-Google IMAP servers or from Exchange. The only non-Google e-mail servers that Gmail can access are POP machines, and you don't get any mailbox synchronization with that method.
First spotted on Download Squad.