I just thought I'd add that i've been using routertech since version 2.1, trust me when i say my dlink does NOT like upgrades through web UI. heh. And based on all these other posters here. I think it's reasonable to assume that it's pretty much impossible. You pretty much have to force feed these things firmware. Just hope chief gets one to test one day to see why they are so dang stuborn. I'll have to force feed 2.6 when i get home. Unfortunately do to my windows machines being vista only, pc tool and the ti tool both fail too.(but they are good for purposely bricking). So i will stil have to purposely brick it. Then use the OS X command line to force feed firmware into adam2 default ftp. :\ a good 30 min process.
Update, just got home, webui failed as usual, reported image had wrong CRC and was not usuable, oh well. (even though it was right image, and the mtd's and config were all right too.).
tried to use darkwoof tool, as usual it reported an "access violation" error and could not update firmware, set it to force, it would hang at 5-10% and brick router as expected. quite routeen at this point. Fired up back into OS X, opened unix ftp command line, and worked adam2 ftp magic, force fed firmware right in through that, reboot. all in order now. 2.6 up and running