Absence of evidence is not evidence of absence. That being said, finding WMD is not necessary for this to be a legal war. The war was in response to Saddam's failure to abide by his obligations as outlined in the 1991 cease fire agreement. If he had no WMD, then he was stupid because his noncompliance over 12 years, firing on US and UK planes, noncooperation with inspectors, and providing false and insufficient documentation to the UN made him in violation of said cease fire agreement. Hence, this was not a preventative war - this was basically a continuation of GW1 (I know it's more complicated than this... I'm just simplifying).
The UN security counsel unanymously agreed that Saddam was in material breach of past resolutions and all of the countries that voted for 1441 believed Saddam had chemical weapons - even France and Germany. I think it is more likely than not that he had them, and if he had them then I hope we find them. There is no reason to believe that these weapons will be easy to find without help from people who were in the know in the former regime.
There have been so many things discussed in the media, by the government(s), this board, etc. about this war (WMD, humanitarian issues, oil, corporate contracts, etc.) that I believe the reasons why this action was taken have become clouded. Whether this action was the right one, though, remains to be seen.