fork() emulation in 5.8 is actually using the same scheme as in 5.6 (ithreads). The ithread code in 5.8 is indeed improved over 5.6, especially in the area of regex concurrency problems. But a lot of XS code doesn't really know about ithreads, and as long as modules are not thread-safe at the XS level, there is not much you can do at the Perl level.

Note that I'm not saying that I think ithreads is working flawless in 5.8; I think it should still be considered experimental.