Nokia's European site says it's compatible with devices 90-130mm long, 50-70mm wide, 8-16mm thick, and up to 200g. The 900 is 127.8 x 68.5 x 11.5 mm and weighs 160g, so the CR-123 should work.
I'd prefer something with a little more reach, though.
The credit does make it very tempting, but I think I'm going to have to wait for Tango (or beyond).
I use my phone to track workouts (several times a week), and the lack of support for BT heart rate monitors is killing me. Sure, I could use a separate HRM, but I need that data into the workout...