Thanks to Volker Kuhlmann, there are now 3 brand new avr-gcc packages in the CrossToolchain:avr repository: avr-gcc-436, avr-gcc-462 and avr-gcc-47-20111105.
The "original" cross-avr-gcc package is still at 4.3.3, as that's the most tested revision and also it has additional patches that are dropped from the new versions. Those patches add, amongst others, XMEGA support which is not present in the avr-gcc packages. That's something to improve in the future.
The packages contain a run-avr-gcc-XXX wrapper script (substitute XXX with the 436, 462,...) to make testing the respective compiler very easy.
Have fun testing!