IDs for nodes/ways/relations have to be 64 bit. 32 bit node IDs are expected to run out somewhere between the first and second quarter of 2013.
Also there is no upper limit for any ID as they will be never re-used.
The most-recent discussion on the 64 bit node ID issue is this one.
Update: Just for the record OSM now has node IDs larger than 32 bit since February 2013.