Ryan,
While my environment is totally different (retail), I have looked at the number of sessions I typically see by devices and it seems to be in the 10-15 sessions per mobile device, with my iPhone having 20 sessions established most times due to several push accounts.
With a safe margin of 30 sessions per device you would likely have no issues with at least 1000 devices per IP address. At my companies retail stores we have 1 address per /21 and I have not run into any issues with running out of sessions, however my average utilization (during business) on these /21s are in the 400 device range with peaks at high traffic locations hitting 800-900 devices, again never having any issue running out of sessions.
If you plan at peak times to have your subnets at high utilizations at peak times I would likely do 1 per /23 or /22. I think us doing /21s pushes it, but not my call.
Too bad to see osuwireless needing to move to RFC1918 space. I'm a big propoent of global addressing. Maybe make best buds with some guys at MERIT and get a couple of /16s from their /8? ;)
Matt