Firstly, you should consider to create a disk partition where both OS'es are able to write data to. NTFS has at the moment only read access in Linux. So, make a shared fat partition or install ext2-driver for Windows. Hereby you can share GAIM data.
Secondly, I don't think it is possible to share pst-files on-the-fly. You can migrate you data from linux to win or back, but it's not real sharing, right? You will need to export and convert data every time you change something. You can read more here http://www.linuxquestions.org/questi....php?p=1848761
My advice to you is to use a calendar program, which is available for multiplatforms. For instance Mozilla Sunbird. Then you can store profiles on the shared partition and open them from any OS.