You can already see the initial wave of it with less people signing up for weekly raids and established raiding guilds beefing up their recruitment. Not to mention established raiding guilds announcing they are folding or halting raiding. It's something that happens every Summer, but seems to be starting somewhat early this year. Surprisingly early considering Ulduar has only been out for a number of weeks.
I think part of the problem is that Ulduar is considerably harder than Naax was and people are in shock over it. My own guild is having a hard time of it lately after making very good progress up to Auriaya, with multiple wipes throughout the evening the norm. And I think that has begun to wear on us as a group. between the allure of nice weather and the grind that Ulduar is over Naax I can see why people might decide to be somewhat scarce these days.
Blizzard has already responded in previous patches by nurfing encounters and abilities in Ulduar but it's still a fairly difficult place overall. Whether less hardcore guilds survive through the summer or continue raiding is a question that only time will tell.