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None. you can get sued by your current employee. don't help any previous employer.

Not always, signal9.

Certainly, that's a relevant bit (which some of the comments on the blog post pointed out), but unless you're working for a competitor, few bosses would begrudge a few minutes of your time chatting with somebody you used to work with. Surely they don't think you'll never talk with your old buddies again? And that the conversation won't turn to "Damn, we've really been having a hard time getting the FooBar module through QA" or other shop talk?

How much a "few minutes" might be is a big question, though. As is the nature of the help, and the way that you're approached.

I agree with signl9. No way. You stand a chance of being a target for a law suit from both your current boss and your previous employer.

You're skipping over the fact that you can actually *communicate* with your new employer. Why freak out because of something they *might* do? Ask them if there are any issues.

As for the time commitment, I'd say 5-10 minutes should likely be gratis. Even if you hated your old job, you probably still want these people on your list of references, don't you? If they need more time (or if they keep asking & asking for little bits of time...) then they should be either:
* paying you by the hour, with an upper limit, or
* contracting you from your current company
...and obviously either these would require written permission from your current employer, as it probably would require an exception to your NCA.

I actually wrote something similar (http://www.dennmart.com/2008/04/19/professional-dependencies/) that's happening to me. It's good to be able to help your previous employers, as long as you both ended the professional relationship on good terms. But most of the times, I've seen it evolve from "Can you spare a few minutes and tell me where the code for this is?" to "Can you spare a few hours and fix this code?" in a few steps. You become a crutch for them, because if they see that they can come to you when they have problems, they will do it - constantly.

Bottom line, while it's good to help out your previous employers and co-workers, have a short limit on what you can actually do for them. The last think you want is to have the relationship, which we assume was on good terms, get spoiled rather quickly.