Elliot Spitzer has been such a sanctimonious prig that everybody who has dealt with him is happy to see him go down. Which testifies to the brilliance of going after him--nobody's going to stand up for him, and probably nobody is going to look into how the evidence against him was gathered.
My guess is they were tapping his phone and came up with the calls to Emperors Club VIP, then worked back to the payments, and not the other way around. Can I prove it? Obviously not. And just like the question of how CBS got a copy of the Texas Air National Guard memo on George W. Bush typed on a modern word processor, chances are nobody is going to look into it. Too bab. Somebody should.