I searched for this answer so if I missed it please excuse me.

With the new TMO BIS, I am able to poll one Exchange account with BIS through OWA (Outlook Web Access) and I use BES for a second Exchange account. This is really critical to me.

If I switch service to Cingular will the Cingular BIS allow me to use the polling feature for OWA. This is a make or break issue for me.

Thanks in advance.....

It should allow you to do so. It is a matter of setting the OWA as the address you want polled.