First, go read Alex's post, because he makes some good points about recruiting for 25 man raids in Wrath of the Lich King. We now know that in the expansion, all raids will come with a 10 man and 25 man setting, effectively a 'normal' and 'heroic' mode for raiding. While I personally believe this to be awesome, I can understand the idea that this will adversely affect (not effect, I'm reminded) recruitment for 25 mans if people can see the exact same content by just running a 10 man. Sure, the gear won't be as good, but if the starter 10 man gear allows you to run the next stage 10 man, and so on until you finally reach a 10 man version of Arthas, guilds that run 25 man raid content might have a harder time recruiting people to run what is essentially the 'same' content with 24 other folks instead of 9 other folks.
The reason I don't think it will be a real issue (I do think it's worth considering, though, and I do believe it will have some effect on 25 man recruitment) is threefold. Unrelated to those reasons (which are coming up after the jump) I have to admit that this may vary by server: on Norgannon, if anything I'm seeing more new 25 man guilds recruiting and starting up the crawl through Gruul's and Magtheridon, so I may just be working from a glass half full through rose colored glasses state of ludicrous (and heavily over-metaphoric) optimism here.