Sorry for bumping up an old thread, but for the sake of completeness for whoever happens to stumble on this topic...
I like nls's suggestion for the archos, but that option is out for me since (as I understand it) the archos can only play mp3 due to hardware limitation, and also I personally am not too fond of small screens.
The 50-pin devices are out, too, since to get a large hard drive a zif-ide adaptor is probably needed but it probably will not play nice with the limited space in the gigabeat f's, and I don't want to carry a heavily bandaged DAP around.
Gigabeat X is really, really hard to find - and expensive, too.
I end up with an ipod video - for all its inadequacy, an video ipod is so much easier to find than any other rockboxable DAPs. So I brought a 30GB ipod video, a 60GB backplate, and a 120GB hard drive (Samsung hs122jc) - all for the price of less than the current 80GB ipod classic. Threw the parts together, and the ipod seems to work out great - not rockboxed yet since I'll still waiting for some smaller stuff before I "seal" the modded ipod, but itunes willingly "restores" the OF into the new hard drive - and I have a 120GB (or 111GB after formatting...) rockboxable DAP!
EDIT: p.s. someone who can edit the wiki (I'm too lazy to get an account) can remove the "(?)" for Samsung HS122JC - it is indeed a zif 40 drive - sorry if that disappoints somebody