Removing The Global GC Lock: Largest Plausible Number of Threads?

[dsimcha]

I'm thinking about ways to remove the global lock from the garbage 
collector for most small allocations.  I'm basically thinking of making 
the free lists thread local.  Every scheme I can come up with that 
doesn't require a radical overhaul of the current implementation 
requires every thread having a unique ID.  I want to do this as simply 
and efficiently as possible, preferably using dense integers.  Is it 
reasonable to assume that no program will ever need more than 2 ^^ 16 
thread (about 65,000) simultaneously so that I can store these indices 
as ushorts?  If a program creates a lot of short-lived threads, the 
indices will be recycled, so having a huge number of threads 
non-simultaneously is not a problem.