Well, for a "normal" company, I'd say charge no less than $125 an hour, and negotiate from there based on your need/desire and keeping in mind the state of the economy. Still, unless you want to do them a favor, you need to charge enough so that you're not burned out (working your ft job then burning the midnight oil / working on weekends). "Doing a favor" rates should be more in line with your market hourly rate -- divide your market salary by 2000 hours. Working for less than this is really sketchy, and you should only venture into that territory if there are significant non-monetary bennies coming your way (resume seriously needs the boost, etc).

I charged $150/hr to setup a VOIP server, and $40/day to teach Scheme. What could you possibly triangulate from this?

Just know your industry and ask your peers.

Ye, that helps thanks. Any more data points?

Why don't you call Dell, IBM, Sun et al. and become a sales partner? it's the IT equivalent of street-corner pushing and you will learn how much things are worth IRL very quickly.