It's a factor and IMO it's a factor which will play a larger role as Blu-Ray movies take off further and the HD DVD camp admits its defeat (a reason why Microsoft may push the HD DVD camp to carry on a little bit longer to produce misinformative analysis of the market.)
However for the PS3 Blu-Ray is mostly relevant with regard to gaming, no disc scratching, much less drive noise, good solid constant streaming speeds and a lot more space for developers to work with.
With regard to those for who Blu-Ray movie compatibility is the main factor for buying the console, IMO the bulk of them will investigate the PS3's other abilities and buy games for themselves and / or the rest of their family, maybe not as many as a hardcore gamer would, but IMO don't write such people off at all.
Many hardcore gamers will buy a PS3 when GT5, MGS4, GTA IV, FF XIII, KZ2, R2, etc, etc hit the market.