I've upgraded my MacBook Pro from Tiger all the way to Snow Leopard so here are a few observations.
1. Heasoft builds and runs successfully using gcc from Xcode (4.2.1) and gfortran from the R project Mac installer (http://r.research.att.com/tools/ : 4.2.3).
2. Heasoft builds and runs successfully using gcc from Xcode (4.2.1) and gfortran from the fink-installed gcc44 package (4.4.2).
3. Heasoft builds but xspec crashes using gcc and gfortran from the fink-installed gcc44 package. The problem appears to be conflicts in exception handling perhaps involving the system X11 library.
4. The iStat Menus application can be used to monitor usage of individual cores. I find that when running compute-intensive processes (such as xspec) both cores on my laptop get hammered. This implies that Apple, at least, are performing some parallelization in the compiler without specific user directives. This may depend on optimization settings.