Aside from the infamous incompatibility of VPC, the only application I've found so far that doesn't run on my dual G5 is Microtek's ScanWizard software, which I use with my Microtek USB X12USL scanner. Shortly after it launches its OCR module, the OCR module crashes, which in turn crashes ScanWizard. I'm using ScanWizard 5 v. 7.10, which is the one currently on their Web site.
Is anybody else running this successfully on their G5? If so, how much RAM do you have? I wonder if the OCR module gets confused because I have 2.5G RAM.
FYI, Nikon Scan 3.1.4 works fine on my slide scanner.