Well, lots of people are using 10.5.7 without issue, so it seems like it is something with that model...
Apple will sometimes ship beta-ish OS builds with their newest hardware. What is the build version of the Leopard that came with your laptop? You can check this in System-Profiler by clicking on the "Software" subcategory(not an item in it).
Mine is 9J61, which was displayed as "Mac OS X 10.5.7 (9J61)". It looks like the Macbook Pros made after June use build 9J3050. If both of you have this build, then I'd say it's a kext(kernel extension) problem and FTDI and Apple would like to know about it.