I'm coming to this thread a bit late, but here goes...
The DSL-300T modem is a crippled router. As can be seen from the review article linked to in other posts, it is based on the TI AR7 communications processor. The DSL-300T is actually the same as the DSL-500T single port router, and can be changed into that model by loading appropriate firmware.
I've not personally done it, but many others have. There's even a healthy 3rd party firmware interest in this, as a quick Google will tell you. Much of it is based on other companies' offerings with a few tweaks. The following is probably a good starting point, but I can provide more info if needs be:
http://ar7.wikispaces.com/Acorp+modified+firmware+for+D-Link+30xT
Kind regards
EDIT:
Just had a quick look at the pic in the review, and I though it worth (?) adding a bit more info.
If you look closely, you can see that the circuit board has the legend:
Annex A
Annex B
Modem
Router
Modem + USB
Router + USB
and that 'Annex A' and 'Modem' are marked with a red blob, confirming that it's a DSL-300T.
On the DSL-300T model, the USB port is missing. If it were present, it would be a DSL-302T. With this exception (the USB port), the other options are firmware controlled. The router is the DSL-500T or 502T. The Annex B options (ADSL signals at frequencies above ISDN) are the DSL-360T, 362T, 560T, and 562T models and Annex A models can be 'modified' for Annex B operation by loading the appropriate firmware.
There is also a JTAG connector (top right in the pic) which in addition to the ADAM2 boot monitor which in itself makes the AR7 almost crashproof, allows the product to be reloaded with a new bootloader should ADAM2 ever be corrupted. You can even replace the boot monitor with another (PSPBOOT, say) should you feel the need.
This is actually a pretty good product: it runs an embedded Linux variant and is an enthusiasts' paradise, if you like that kind of thing...
Edited by rperkin (Sat 02-Jun-07 02:16:58)