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Regardless of getting VPN to work, note that Apple is about to release Lion (10.7), and will likely drop support for 10.5 (while they have not stated that officially, that has always been the way Apple does things; two versions are supported, otherwise you need to upgrade). And, as you know, the PPC/G5 is not supported in 10.6/10.7, so you really need to plan to replace that system. When Apple releases security fixes for 10.6/10.7 and not for 10.5 (figure August-ish, but that is a WAG), 10.5 will no longer be in compliance to connect to the internal SLAC network.
The Cisco AnyConnect client for PPC is stuck at version 2.5. Although Cisco has been releasing security patches for that version, it is one major release behind the current software for all other platforms, which is at 3.0.
We performed significant testing of the 2.5 client, including on PPC systems, but eventually we made the decision to support only 3.0+ client versions due to the small number of users who would be affected, and the assumption that Apple will not continue to support PPC systems with OS updates for very long (we don't know for sure, but are making an educated guess based on Apple's past behavior).