No. Well OK just maybe, but as I understand it spawning a new thread means copying all memory structures (kinda like a fork), which means growth in memory use. (Even with copy on write, because you have to spend a page to toggle a bit)
The advice I've read is to spawn all your threads at startup time when the memory footprint is low, and that gives me an icky feeling about the whole thing. I hope Perl6 will include Java-like threads (only good).