I thought I would share my experience with this modem, in the hope that I might save others some grief!
I wanted to setup the following
<Internet>---<Linksys AM300 192.168.0.1 f/w 1.19.04>---<WRT54G 192.168.1.1 f/w DD-WRT v24>----<XP PC>
In half bridge mode the modem shoudl terminate the PPPoA connection but pass on the real world IP to the next device in the chain.
The inital problem was that I could not get the AM300 to pass its real world IP onto the WRT54G!!
I dont quote know how I fixed it, but I did. The only problem that now remains is that if the DSL Link is not up, no IP is passed onto the WRT54G and hence I cant get into the AM300 setup pages on 192.168.0.1.
The funny thing about this is I got the linksys to replace the Dlink 502T in the hope that the linksys would be better. My synch speeds increased from 14800Kbps to 15500kbps, but other than that there is no improvement (VF Red Network).
Also the AM300 seems to take longer to connect, but it does have a PPP light which is handy.
Also, if your modem in half bridge mode disconnects and reconnects it gets a new IP, but many modems dont pass this on to the router, you have to wait until the router lease time expires and it reconnects. If you are running DD-WRT you can get a script that checks the IP gateway and will renew automtically if it cant ping the gateway. Look at http://www.dd-wrt.com/wiki/index.php/Useful_Scripts
Hope this info is helpful for people in the future. The key point is not to try to force the router to get its IP until the AM300 link is up!