I'll admit this isn't a life-changing tip, but a reminder that Google Talk has been redesigned specifically for the iPhone. It runs right in your Safari browser, so you don't have to download any special application. All you have to do is navigate to talk.google.com and sign in to your account.
The version was made to work in your browser so you'll be automatically signed out of Talk when you navigate to a different browser window or app. That means you won't always be online, but you can check in at any time to see if your friends are online or if they have updated their clever status updates.