i haven't heard of any issues with opening attachments with Comcast accounts
What T-Mobile is recommending you do is set up a forwading rule on your Comcast account to forward your email directly to a tmo.blackberry.net address that you create.
In doing this you will get your email right away and any attachments as well. however unless you change the "reply to" address for your BIS address to your Comcast you will be sending email and repling to email using your tmo.blackberry.net address
The other option is to add your Comcast account to your BlackBerry via the T-Mobile web site. This way your email is polled about every 15 minutes. You won't need a tmo.blackberry.net address and when you send new email and reply it will automatically use your Comcast account.
You mentioned syncinign without duplicates - when you sync with your laptop your email will not be affected when your on a BIS - only a BES
i also found the following aritcle on RIM's public kb that lists what attachments are supported
http://www.blackberry.com/knowledgec...22744&vernum=0
I know there are more (like jpeg) now supported with BIS 2.0 (which T-Mobile is on) but can't find an updated article
Hope this helps - and enjoy your BlackBerry