Since I saved a lot of the layers while working on this I thought I might as well put it up.
Fairly straightforward process, really - the first colour pass involved two layers set to multiply, just painted over the top of the first one, and after that it's just a matter of actually painting.
The painting is all done with a combination of these three brushes
, mostly the second splodgy one, which is nice for soft smooth blending while keeping a sort of 'painterly' look at the same time.
The final piece is 6000px wide, and the whole thing probably took 8 or 9 hours, in PS CS4.
The final is up here -