If the SBS 2k3 is handling DHCP, it should be able to change the gateway for everything whenever you need it to... but it sounds like the option of sharing the 3G via ICS and a router would be the way to go. Then you shouldn't need to change anything on the server.
Is the WRT54G already set to obtain an (outside, or WAN) IP automatically from the ISP?
Do you have a PC (with a cat5 cable that runs all the way to the WRT54G,) in a location where you get good signal with the 3G adapter?
If so, that's the one I would use for the ICS machine. Install the 3G adapter, and (from Network Connections, or Internet Options on the Connections tab) in its Properties on the Advanced tab enable ICS and choose the ethernet adapter from the picklist (if the PC has multiple network adapters; otherwise the ethernet adapter will be selected automatically). When you click OK, the ICS setup will disable the windows firewall on the ethernet port (so have it turned on if you're planning on using the firewall) and it will set the ethernet adapter to the static IP of 192.168.0.1.
Next, unplug the cable from your current internet connection feeding the WAN port of the WRT54G, and replace it with the cable coming from the ICS box's ethernet port. If the WRT54G's WAN side is setup to receive its address automatically, the ICS (ethernet) adapter on the 3G box should assign the WAN port on the Linksys 192.168.0.2 or 192.168.0.3 (with the gateway of 192.168.0.1), and the router will handle all NAT'ing between 192.168.0.0 and 192.168.15.0 while your SBS goes right on handing out the same scope and gateway as always.
Once it's working, you don't need to uncheck the ICS setting for the 3G adapter when you're not using it - just disconnect the adapter from the ICS computer. So there are 3 steps to switching back and forth...
To change the ICS machine back to 'normal' duty, swap the cat5 from the WAN port back to a LAN port, change its TCP/IP back to 'obtain address automatically' and enable the firewall on the ethernet port again (see attached).
To switch back to 3G again after that, set the TCP/IP back to 192.168.0.1, UNcheck the box next to the ethernet adapter in the firewall, and move the cat5 from LAN to WAN port.
i.e. if you want/need to switch back and forth; If you're staying on the 3G, you don't have to worry about those steps.
Oh, and if you want to setup a manual connectoid for the 3G instead of using a supplied connection manager, it's much like creating one for a POTS dialup modem, except you don't need a username or password (activation takes care of that), and all the GSM/UMTS/HSDPA cards I've seen used either *98 or *99 as the number to dial (sprint/nextel/verizon all use #777 for CDMA/EVDO).
I'm sure I forgot something, so let me know what I missed when you run into it, or what I didn't explain enough. :-)
