I ran my Nexus One off the unlimited $40/mo data-only plan for its first two years and saved around $USD2000 vs buying subsidized, but it appears that they dropped it in favour of the $50/mo plan. You'd still save a bundle vs the subsidized phone+full plan over the life of a 2-year contract.

I'd always get the weirdest looks when I went in to a T-Mobile shop since I was grandfathered in to the old plan and they had never seen it, so that was always fun.