Hey, I'm working on slimming down and getting buff for my wedding next year. I too have a desk job, though it's my own business. Even when you work for yourself you don't have time!
One thing I read about in one of my Bridal Mags is something I plan on starting next week. Basically, this girl toned up by lifting a dumbell while on the phone at work. Anytime she had a moment and a free hand, she used her free weights! Sounds like a good way to tone up when you don't have time, and who knows...maybe you'll look forward to phone calls and breaks.
As for actual aerobic activity, I do tapes on occassion, but mostly I walk in the mornings with my puppy!
Remember that 5 minutes is better than 0!