I would strongly recommend those USB adapters (probably the one Tony recommended - it looks less likely to snag on your laptop bag if you keep it plugged in), but if you're set on PCMCIA, I have one of
these. The only protrusions are the parts at the bottom, one of which is the tip of an antenna that you can pull out for extra range (I've never needed that within six feet, though).
The only problem I have with it is that it doesn't appear to work under Linux. I'm probably just not familiar enough with Ubuntu, but that's what my laptop is running right now, and I'd love to get my mouse working with it. If you're looking for a bluetooth mouse, I
highly recommend
this one. I prefer my black model, but it seems a little harder to find these days. That's the smallest portable mouse that I've seen that is still usable, even for my huge hands.