It also helps to have two bank accounts, one for business and one for personal, when filling out your taxes and just about any bureaucratic form you wind up having to do. The info is all together in one place and you don't risk missing a big deduction for the tax man.
Also I think if you ever have a question come up about whether or not what you are doing is a business versus a hobby, the existence of a separate bank account will help justify your "business" stance.
I guess if you have more than one business going on, it might help to have separate accounts for all of them. We're just starting to get free chequing accounts going in my neck of the woods, but if those are available, it's just the cost of the initial checks (if that even) which can be written off on taxes that counts and the ease it'll bring to your bookkeeping is well worth it, IMHO.