Check out this spectacular crib, headsmacking for both its "why didn't I think of that?" practical ingenuity, and "what were they thinking?" name.
The Staypuff Crib is a managed forest pine crib built on top of a utility wagon. It's awesome, but also an unusual calling card for a firm, 20.87 Design Studio, of Sao Paulo, whose only other identifiable creation is a single, content-free Flash animation.
The catch, of course, is that just because they're both used in places called nurseries, cribs and carts don't share standard dimensions. So you'll be hard pressed to find a mattress that fits as snugly as safety regulations prescribe.
Still, it's probably easier to get a 30x48 mattress made than a 52x27.5 cart. Have at it, and send specs and pictures!
Staypuff Crib by 20.87 Design Studio [mocoloco]
Is there a hidden message in the Flash? I don't care enough to find out. [2087.tv]
12 pages of search results for "cart" [northerntool.com]
Just because you can, doesn't mean you should . . .
Playpens on wheels are very common in Israel -- they're used for walks in daycares (instead of the 6-person strollers used for that in the US) or for parents of twins. Basically they're used as an alternative to multi-kid strollers. The wheels aren't usually that big, though. And I'm not sure whether or not they're equipped with mattresses.
Sounds like The Promised Land to me!
I was going to say the same thing as eli did above. They're pretty common in Israel and the one I saw in downtown Jerusalem did have a mattress in it. It was being pushed by what looked like proud grandparents and seemed to be an older model (from the 70's?) with a bright -yellow, tubular metal frame and no handle iirc. It was great because the 2 toddlers in it could move around, play, watch the scenery or nap whenever they felt like it and weren't confined by safety belts.
Wish I'd taken a picture..