I have a Live account through Hotmail and that is what I use for basically everything but email. I have it set to not sync email because I don't use it for email any longer. But it does manage my contacts, calendar, online services like Office, and more importantly it is the Live account that my Zune Pass and Xbox Live info is connected to.
For anybody who uses a Zune Pass or Xbox it is important to know that you can only use one Live account for Windows Phone 7. There have been some cases where individuals who use different Live accounts for each of these services have had problems because you can't combine two accounts on the phone jointly. There is a process to merge the two if this is a problem you're having (you're being a general you).
As for your case bgoodbody, if your Comcast account is not able to sync calendars and other services like that (but you want to), I would suggest using the Hotmail for the phone but setup a separate email account on the phone for the Comcast email. Then if you don't want to be bothered with any email coming from Hotmail ser it to not sync email. That's what I have done and it's working wonderfully.