This is a long shot, but the 17" model definitely requires the more powerful 65W AC adapter, as opposed to the 45W AC used by less power-consuming models. Any chance that you are now using a different AC adapter, that that it is the wrong one? Using a 45W AC on a PowerBook 15" or 17" sometimes produces behavior like you mentioned, if there are any signs of life at all. I've had many machines sold to me as "dead" only to find that the wrong AC was used.
If that's not it, I'd remove the battery and any extended RAM, them reset the PRAM and PMU, and see if any of that makes a difference. Is there a solid light on the AC adapter plug, or is it not showing a light?