Tunneling protocol(s) enabled in the concentrator, and transparent tunneling settings in the Transport tab of the client have everything to do with whether a particular config works or not.
Particular networks may not permit some tunneling protocols (L2TP, PPTP, IPSEC), and firewalls are a major issue as clients may not be configured to work around some firewalls (NAT-T {IPSEC over TCP} vs IPSEC over UDP).
I have had no problems with 4.9.00.0500 with my configs with either Intel or recent PPC macs in 10.4.7
Check your client config, or better yet get a config file from the VPN admin.
PPC or Intel Macs Mac OS X (10.4.7)