If anything, they'll be able to support 3G/HSPA+ speeds for the iPhone due to their refarming of spectrum.
However, I would not want to get an iPhone through Tmobile if they were to sell it. I enjoy my $5 a month data plan...of course, this could easily go away. But Tmobile USA twitter said that after refarming is complete, iPhone customers could keep their existing unlimited data plans.
Of course, this could change too. But until that changes, I would keep using an iPhone out of contract and with my $5/month data plan.
(And personally, I go out of the country a few times a year so an unlocked phone would be far more useful for me)