To access Google Talk for mobile, just visit http://www.google.com/talk from your iPhone device or go to http://google.com and choose Talk from the More tab.
If you are a Google Apps user, you can access Google Talk for your domain by following these steps:
- Go to http://google.com/m/a/MYCOMPANY.ORG but replace [MYCOMPANY.ORG] with your domain.
- Tap the More tab.
- Tap the Talk icon.
Tip: Bookmark the Google Talk URL in Safari by tapping the + button.