Last week, Google launched a new version of Google Talk for mobile, optimized for iPhone’s browser. It works right in your Safari browser, so there's no need to download or install anything -- just go to talk.google.com and sign in.*
Because this version of Talk is designed to run in your browser, you'll be automatically signed out of Talk when you navigate to a different browser window or iPhone app. So while it's not a traditional "always-on" instant message client, it's useful for changing your status message on the go or checking in to see if someone's online and sending them a quick chat. It's equally nice for killing time at the airport. [Gmail Blog]
*Google Apps users: visit http://talkgadget.google.com/a/your-domain.com/talkgadget/m from your iPhone's browser, but be sure to replace 'your-domain.com' with your actual domain name.