Yeah, it would have the latest firmware, but apparently the older 54G's had more RAM and Flash memory than the newer ones, preventing full installations of 3rd party firmwares like DD-WRT on newer models. The WRT54GL is sold as a "high end" model with the higher RAM and Flash memory, I'm guessing as Linksys' way to profit off of supporting other firmwares, as there doesn't appear too big of a difference between the latest G and GL versions.
However, the 150N looks promising, it has multiple radios and antennas (MIMO) for long connections, Wireless-N, and also full unofficial firmware (DD-WRT) support. For only $8 more, I think I should go with that one. Only thing it's missing is Gigabit Ethernet, but none of my current PC's support that anyways, so no big deal.