Android - Use Google Talk with multiple accounts

EDIT: Looks like this is coming, according to Google employee Colin Gibbs, on this Google Mobile Support forum thread:

Thanks for being patient while we work on this feature. When we launched Android 3.0 (Honeycomb), we added support to Google Talk for multiple accounts. We're currently working on bringing the feature to phones as well.

So it looks like it should work on a Honeycomb tablet, and they may be backporting it to Gingerbread phones (video chat over GTalk has just been enabled for 2.3.4, so they're definitely porting Honeycomb Talk's features back to phones).

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Original Answer:
Not a great solution, doesn't do all the alerting and notifications in a way that's integrated with the Android OS, but the only way to do this currently seems to be to use the built-in GTalk client for one account, and access the other via the web page.

For a standard GTalk account use this URL to sign into the mobile webpage: http://talkgadget.google.com/talkgadget/m

For a Google Apps Gtalk account use this URL to sign into the mobile webpage: https://hostedtalkgadget.google.com/a/YOUR_DOMAIN_HERE.COM/talkgadget/m

Or use a second IM client for the second GTalk account, as suggested by Matthew Read.

As you've pretty much said, the official Google Talk client doesn't have any way to select an account for it. Unless you want to be constantly resetting your phone and changing the primary account, you won't be able to use it with multiple accounts. You can of course request the feature; see the comments below.

I recommend using eBuddy Messenger instead. It's the best IM client I've used so far on Android, and easily handles multiple accounts regardless of which service(s) they come from. Just be careful to sign out properly; if the app is force-closed, Google never clues in to the fact that the client is gone and you'll still appear signed in to your contacts. (I have no idea why it handles this fine with every other client).