Not exactly.....
This happened to me a couple of times, I finally emailed support on one site and they explained it.....If you raise say 200, and then someone goes all in for their last 300 chips then another person calls the 300, when it gets back to you, you can ONLY call. This is because the all in player did not make a full raise (+ another 200 chips) and you would be essentially raising yourself. I am not sure if that other caller after the all in could have raised (I think they could) but you cannot having already raised and not been officially re-raised.
EDIT....Drew...In the case you mentioned, the all in player didn't "raise" the bet so you couldn't raise either. This might answer my question above about whether that other caller could have raised. It is a very weird rule all around, but the way they explained it to me, it did make sense at the time.
This sucked for me because I wanted to put more pressure on that other caller with the best hand, but one that was vulnerable to draws at the time. I can't remember how it turned out, but since I was motivated to email support to find out why I couldn't raise in this situation, it probably wasn;t good.
EDIT--And I just read the second question too. If you sit out and the blinds pass you by, you either pay the blinds you missed or wait till you are in the BB and post then. This is sometimes 1BB and 1SB (or 1 1/2 BB).
If you sit in at a table and don;t want to wait for the BB, you have to post the BB and then post the BB again when it comes around to you.