This is (almost) fully supported. The onboard switch PHY for the Rev B1 is not yet supported and acts as a dumb switch. The Rev C1 is fully supported, ethernet switch and all.
Rev B1:
Rev C1:
OpenWRT has a good writeup: http://wiki.openwrt.org/toh/d-link/dir-825
Adrian's build scripts build a basic firmware image for this platform: