Media Net Live?

I recently bought a v557 with this new feature from Cingular. My question is will it work if i use an att sim card but a Cingular phone?

Nope, as it's not even offered on any of the AT&T plans.

Medianet is Cingular's version of Mmode. Two different services.
And if you do not have a Cingular account of any sort, you'd not be plug in all the MediaNet settings and get it working, as long as you were using any other carrier's SIM, other than Cingular.