For Remember The Milk, there is this application, done in the Google Summer of Code.

If you just want to see how it works, go here

About Google's services:

• Gmail: There's a gmail notifier, which, as far as I know, can only handle an account. It lives in the status bar, and changes its color if new mail arrives. You can also use a Mail client, but I can't tell you about it. I've tried the one which comes preinstalled, Modest, but I wasn't too happy with it. However, it's better here that you wait for another, more optimistic, opinion.
  
• Google Calendar. There's an application, GPE-Calendar, which can sync and show your Google Calendar using Erminig (another app). I'm not using them right now, so I don't know if there are any issues that may concern you.
  
• Google Voice. I think that there's a Google Voice client, but I haven't tried it yet.
  
• Google Docs. As far as I know, it worked fine from the browser. But I don't use it, so I may be wrong.

I use the Tablet mostly for browsing, for reading feeds with Google Reader (I use the iPhone optimized web app), and for logging remotely with ssh. I also use it for multimedia, and reading ebooks.

As a final thought, I think that this device has a lot of potential, but had little support from Nokia in several fields, mainly PIM. However, the effort from its community has improved the tablets a lot, and new software is always coming to fill people's needs. The drawback is that you've got to expend some time if you want the tablet to do things which aren't there "out of the box". It will probably be able to do them, but you'll have to put hours.

In the other hand, the iPod may work better for your needs right out of the box, but has less potential "on the long run" (if there is that "long run" :P)