[anthonyrubin]

I think the problem with a domain like charge.37signals.com is that many users don't understand how to type it in the address box (e.g. some will add www before it). If you've watched users try to enter URLs with subdomains in the address box you've likely experienced this.

[amobilebiz]

Good point, I didn't even think of that and it is true. One option would be to add a CNAME entry for the www portion on the subdomain. That way if someone did it would route correctly.

[jskopek]

They could also purchase the full domain name (charge37signals.com) and have it redirect to charge.37signals.com; that way they account for all possible permutations.