How to obtain the requested category with permalinks in effect?

I need to gain access to the currently requested category (either by id, or nicename or whatever) for use in plugins and side bar.

I was able to do this under WP1.5, but things appear to have changed under WP2.0 and when permalinks are running I haven’t a clue how to now obtain the currently requested category as a variable.

What’s changed and how do I get access to this needed variable? Been looking half the afternoon already and it’s probably staring me in the face, but I’m sure not seeing it.

TIA

Rob