I had a similar problem, even though I'm still on Mac OS9. While it was possible for me to use Avantgo, it also meant I that could only access it by using the ancient Palm Desktop 2.6.3. When I had to move to Palm 4.0 in order to run other necessary applications on my Clie, I was forced to leave Avantgo behind.
I agree, iSiloX is much better. Now if people would just all make their handheld-formatted web sites more publically available, rather than trying to hide them from non-Avantgo users ...