Thanks for the quick reply. DVD discs written in the UDF format ( http://en.wikipedia.org/wiki/Universal_Disk_Format ) should be able to have several sessions written to them, at least with some revisions/variants. However, according to the same article, Mac OS X 10.4.5 will not mount UDF discs with a VAT, and the only way to bun multisession DVDs seems to use UDF with a VAT (don't ask me what this means) ... looks like I answered my own question :-(
My problem is that a former Windows user has asked on Apple Discussions ( http://discussions.apple.com/thread.jspa?threadID=419215 ) why he couldn't write multisession data DVDs on his new PM G5, and claims that he was able to do so on his Windows PC.