I warned Fiance the first time he came to my apartment when we were first dating--don't leave anything unattended that you actually want to drink. ;) My cat does the same thing! She'll drink right out of the glass if you offer it up to her, and she won't wait for an invitation if no-one seems to be offering one. I don't share back-and-forth with her, but sometimes I'll let her have a sip of my water before I take my glass to the kitchen.
She also sticks her paw in her own water dish and licks it off, and she loves to drink the water in the bottom of the tub after I shower. I tried to discourage that (because EW) but she doesn't want to give it up. I figure if it was all that bad it would taste like soap, so I just let it go. I draw the line at the toilet, though. The first time I caught her sticking her paws in the toilet was when I started leaving the lid down religiously. ;)