The new firmware can be installed over the VxWorks operating system found in the Linksys WRT54G version 5 router: This router caused some irritation among those who wanted to use it with modified firmware that worked in version 4 and earlier, which relied on embedded Linux and contained double the volatile and flash memory. Linksys says it shaved costs on memory because the Linux they used couldn't be shrunk enough. (For more on this, see this very long set of comments on a thread I started after having reproducible, continuous problems with two newer Linksys devices that use VxWorks.)
Linksys did recognize the interest among the open-source and community wireless movement, and added the WRT54GL to its line-up, which is essentially the WRT54G v4--and which sells for about $20 more than the v4 and v5.
This latest wrinkle allows a non-reversible installation of a very compact version of the DD-WRT distribution. However, this is clearly a step along the road to more functionality. Making code smaller is always tricky because it often means stepping down below high-level programming languages to optimize inefficient libraries, which in turn requires much more testing and is harder to debug. (Take it from a guy who cut his teeth on 6502 machine language.)