You can forget about the fax machine- the 6610, or any other T-Mo
can't possibly interface with it.

As to the PC, it's possible but akward.  T-Mo offers a fax plan for
$10/month (you get assigned a second fax number so the phone can
distinguish a fax call from voice by what number is dialled, then
everytime you get an incoming fax, you dash to your computer and
hastily tether the phone to it.  Good luck!

Simpler just to use T-Mo's Faxmail, and is free.  When you get an
incoming fax, let it go to voice mail.  T-Mo actually receives the fax
for you, alerts you by SMS, then you call your voicemail and follow
the prompts to have the fax resent to the fax number of your choice.

Or, do what I did- get a free account at www.faxwave.com.  They are a
fax-to-e-mail gateway- they give you a fax number in Nevada (you can
pay a monthly free for a local fax number) and everytime you get a fax
to that number, they e-mail to you as an attachment.  I can pick those
up with my PocketPC tethered to my T-Mo phone, no computer or fax
machine necessary.

Outgoing is tougher- you either need an e-mail to fax gateway like
efax or jfax, or you have to have the $10/month fax option I mentioned
earlier to send faxes using your 6610 as a faxmodem.  (Via IR, at
least- I don't know about with a data cable.)