IMHO, if you do a startup, do it as a founder, or join after Series B, from a pure risk-rewards perspective.
You should seriously consider writing a "Don't Let This Happen to You" piece (a la Philip Greenspun) and posting it here to save others from the same fate. Change the names (including your own) if you like. A post like that could be the single most important thing some hackers ever take away from here.
Although on a cautionary note, I'd be careful what I post here, because it does tend to get picked up. Comments I've made got picked up by ValleyWag, and that's almost never a good thing.
To be fair if you were an employee, i.e. you got paid well I assume? there was less risk on your part relative to the founders. So your equity should be a fraction of what the founders took in. Why would it be otherwise?
I was responding to ojbyrne's comments that being a founder is not all hunky-dory since (s)he ended up in the same spot. I suggested at least (s)he got better equity being a founder and that is a big deal.
And I still think for a risk/reward outlook, either being a founder or playing it safe until after Series B are the two extremes and are both high in the value / risk ratio.