CONCLUSION
As a last 'hail mary pass', I decided to re-install pfSense 2.0 from a cd instead of using the 1.2.3 upgrade that was in place. Within a few minutes, I had the IPSec mobile client tunnel configured and had connectivity to the protected LAN with a Shrew Soft client. I can't say why. I used all the same IPSec parameters on the server and didn't touch the client parameters at all.
Here are some other things I learned along the way:
1. UDP500 outbound NAT must be statically mapped. Since this is now automatically generated, it is not an issue.
2. The IPsec implementation does not support multi-WAN failover, and gateway failover groups cannot be used in IPSec firewall rules. This is unfortunate. Giving it WAN failover functionality would make pfSense an excellent replacement for the Netgear FVS336G Dual WAN VPN Gateway my customer is trying to move away from.
3. SDPs can have mismatched ports after a change to the IPSec configuration. This will prevent traffic from passing through the tunnel. Restarting racoon clears this up, but is cumbersome.
4. Even though the Shrew Soft client works, the Netgear client still can't obtain an automatic virtual address. However, if configured to, it can request and obtain a specific one. Nonetheless, even though it and the server report an established tunnel, there is no connectivity with the server's LAN address or any computer in the LAN. I'm abandoning the effort, though. My goal is a working IPSec tunnel with an Avaya 4620SW IP phone. The software client was only intended as a breadboard and the Shrew Soft client has obliged.
I'll open up a new thread with regard to the phone tests.