Paul McCullough has done a lot of thinking and a fair bit of implementing in
this area, including a no-moving-blocks allocator/gc. As I understand it you
pay a quite high price in overhead to support it.
GC will always cost some space and some time. The differences are in the
balance and I still like the balance of a well implemented object table. No
irritating problems with becomes, for example. No forwarding table hassles.
Reliable hashes. Bit of a pain for really big systems though. 

tim
--
Tim Rowledge, tim at sumeru.stanford.edu, http://sumeru.stanford.edu/tim
UNIX is many things to many people, but it has never been everything to anybody.