Portland has one, so why don't we?
You are a web developer. You need to develop more on mobile devices. Buying a bunch of devices to test on is expensive, confusing, and annoying. Emulators won't cut it. You wish there was one place to go to test with a large variety of devices.
A Device Lab is just the ticket. Companies can "subscribe" to the lab for complete access when they need it. Dues go to getting more devices or data plans (because testing with WiFi is one thing, testing on 3G is another).
Who'd be interested? Better yet, who'd be interested in running it?
More info about the Portland one: http://mobileportland.com/device-lab
As the founder of MoDevRI and a mobile developer myself, I like the idea, but that's a serious investment in hardware and wireless fees. You'd have to be absolutely sure that you'd have enough people using it monthly.
I like what Mob4Hire does; they crowdsource testers with a wide variety of devices, and then a developer can launch a test to any variety of users.
Gyuri: Excellent point regarding the hardware and bandwidth investment. I wonder how MobilePortland handles this?