Thanks David for your quick answer. I won't be at work until Monday so I cannot try anything on that particular setup. On my system (Pentium D 925/Win2k), arduino.exe works like a charm but running run.bat gives me exactly the same error as described above. So I thought I might try to get it running from run.bat on my system but I've tried the corrections you suggested and other combinations but so far, nothing works.
On the other hand, my machine at home has a dual boot with Ubuntu 6.10 (64 bits) so I could give a try to compile Arduino from scratch. I followed the method given on gerlad's weblog and, except the fact that I had to symlink to the 64 bits version of librxtxSerial.so, the environment starts.
The problem comes with the USB serial transfer. The board is correctly recognized by the system and /dev/ttySUB0 device was created. I built Arduino the first time from svn sources, getting a 0008-alpha version. The environment works, compiles but uploading does nothing, no communication. Then, on gerald's weblog, he starts from the 0006 source tree so I did the same and under root, everything works. I still have to see how it will goes under a normal user.
After this, I followed the tutorial on installing Arduino on Ubuntu, on the Arduino website. Except the fact that I still had to symlink the library (I supposed I should have modified LIBPATH in the arduino script instead), the environment works (version 0007) but the USB transfer is still inoperative (as a normal user and as root).
I'm still investigating the problem, trying to figure out what makes the 0006 in-situ compiled version different from the 0007 already compiled. But I might miss something really obvious...
Thanks again for the great support!!
Stephan