If Blu-Ray becomes the dominant format (which seems likely at this point) then the overall boost to the PS3 from having a built in Blu-Ray player will probably be similar to what the original XBox received from having XBox Live. Blu-Ray and HD-DVD are really nice formats, but they have a very long way to go before they become mainstream ...
Over the next 18 months we will move towards every movie released by a movie studio will be released in an HD-Format, and movie rental places will start to carry a small quantity of all of these releases; this will be a major improvement over the current state where most video rental stores cary a tiny quantity of the most major movie releases in an HD format. Move players will come down to the $100 range and most new laptop and PCs will come with a built in HD-Format drive. At this point most 'average' consumers will be willing to start to consider buying into a HD-movie player; for those on the market for a gaming system the (then) $300 PS3 which has a built in Blu-Ray player will be a good bargin, but for the (roughly) 70+ Million people who will own an XBox 360 or Wii the PS3 will seem redundant and expensive when they can buy the main feature they want for $100.