.Mac isn't an ISP, so you can get it without giving up your current DSL access. Also, you do not need .Mac if all you'll use iSync for is syncing a PDA or cellphone to the Mac.
For most other uses of iSync, however, you do need .Mac. There are ways around this, but they're not terribly easy to work with, and they can cause problems later on if you decide to actually use .mac instead.