Linksys released a firmware update on 4/2/2008 (Version 1.19.04) that lists "Resolves issue with LAN Subnet when using PPP Half Bridge" as one of the improvements made to the firmware. You can get the update here. I've been running the new firmware for a couple of hours now and it seems to be working well.
When running the modem in half-bridge mode I would suggest a lease interval of 5 minutes for the DHCP server. This ensures that if your public IP adress changes you'll be without internet for a maximum of 2.5 minutes.