Let's hope so. They haven't given a lot of indication that they understand the technical details of nuclear bombs - one of their "source materials" found in Afganistan was a reprint from the Journal of Irreproduceable Results for crying out loud. (The Journal was a publication - no longer published - that printed humourous scientific satire). However it's likely that they have at least some members who are more sophisticated and less gullible.
What's more likely is a series of "dirty bombs" - this is much easier (a simple dirty bomb could be built by any reasonably bright high school or even junior high kid who had access to radioactive materials). It would also be in keeping with their usually inflated rhetoric, where the destruction of the Twin Towers was magnified into the conquest of New York. Though it would still be no picnic - a few dirty bombs could cause a huge amount of disruption especially if they dispersed enough radioactive material, but they wouldn't produce anywhere near the destruction of a nuclear bomb or two.
However the clock is ticking for their plans - if they want to claim the mantle of the Mahdi and the new Caliphate, they need to do so soon. Going by the astrological signs that have been claimed for the emergence of the Mahdi, the years 2003-2006 are critical. The astrological signs won't have this good a match for hundreds of years - if they don't make much of a mark soon, it will be that much harder to convince the less fanatical Muslims that they're really the movement of the Mahdi. It is quite clear that they'd very much like to cast themselves in that role.
I wouldn't go that far. They have has little success since Spain. London attacks galvanized public against radical islam and hurt their cause more than helped. Same with Bali, infact the trend world wide is that the popularity of terror is going down. Just on that level alone, given the general success of the WOT, they have to pull off something spectacular to gain the PR advantage again.