Trouble running Mathematica

[Stephen Montgomery-Smith]

When I run Mathematica 5.2 on FreeBSD RELENG_7 I get the following message:

/usr/local/mma52/SystemFiles/Kernel/Binaries/Linux/MathKernel: error 
while loading shared libraries: /usr/lib/librt.so.1: ELF file OS ABI invalid

It worked fine on RELENG_6.  Furthermore Mathematica 5.0 seems to work 
fine on RELENG_7.

I have the usual set of linux emulation libraries installed (what is 
pulled in by building the acroread7 port.)

Any ideas?  I'm really not sure how to go about diagnosing this apart 
from running ldd, which doesn't seem to tell me much more.

Thanks, Stephen