The compiler upgrade is all but complete, and there will be build system changes to match starting with 19. I am strongly considering a formal beta for 19 to make sure we catch all of the widget changes and find any 10.4-specific bugs, as well as the standard 19 unstable release.
I have been made aware by its creator, Misty DeMeo, of "Tigerbrew," a Homebrew port to PowerPC Tiger (our favourite operating system). For those unfamiliar, Homebrew is one of the newest and increasingly popular package managers for Mac OS X (even Mozilla has started using it), but it has historically required 10.5 and I have heard of issues with it on Power Macs, both problems of which hopefully Tigerbrew will mitigate as it matures. While we are ordinarily a MacPorts shop and our workflow mostly revolves around it, Homebrew definitely has some nice features and a Tiger version only increases our choices for maintaining our software (besides, of course, good old standards like Fink, which David Fang still maintains TenFourFox as a package for). You can read more about it at its Github, or, to jump in the water, ruby -e "$(curl -fsSkL raw.github.com/mistydemeo/tigerbrew/go)"
Also, just for your amusement (please don't comment on the bug, this is just for chuckles), enjoy this bug report that we all sympathize with.