I think it will be the Infuse 4G, but its also possible the Nexus S can be announced with it too.
Nexus S code number i9020A is already available in Canada. In Guam, they're already selling i9020A in both white and black --- can you imagine, a white Nexus S? The current US version is i9020T, the suffix being for T-Mobile.
I think its possible that AT&T will carry the SGSII later. The phone has a quad band 3G frequency that includes the ones AT&T is using, making the transition swift to the US. But its not likely that AT&T will announce it right now, lest it will make people hold off on potential Infuse and Nexus S purchases. Osborne effect you know.
An AT&T and Samsung announcement will put heavy pressure on Verizon to release Droid Charge this week. Verizon has to announce Droid Charge soon given that retailers have already stocked it and you don't want them teed off, and you don't want Droid Charge to overlap with Droid X2's and LG Revolution's release. Verizon needs to announce Droid Charge after Droid Incredible 2 and Casio Commando but before LG Revolution and Droid X2 hits the market.
Oh, for those who have owned the black Nexus S, the white Nexus S is a dozy. It has this pearl like finish, and unlike the black model, there is a gritty surface on the back to help make the phone less slippery.