I have a first version 1.8 20inch imac, which had black screen and wouldn't work an external monitor iether. everything else appeared to work and I could read the hard drive when hooked up to my powerbook in Target mode. Checked the capicitors and nothing obvious blown. Checking closely one of the capicitors near the screen output cable (there are 2 sitting together on the logic board here) had a very slight bulge in it. I changed it and voila! screen back and a happy mac user again!
Cheers Jeff
PS - cost of capicitor £0.40 and just my time and plenty of guidance on ifixit!