Apple's iMessage service, their iOS and OS X SMS- and MMS- like service, is currently down for a lot of users. It's been going down fairly regularly as of late, though not for lengthy periods of time. Game Center, Apple's online gaming service, has also gone down or suffered from poor service over the last week.
Everything goes down. Google goes down. Amazon does down. RIM goes down. Microsoft goes down. We go down. But reliability is a feature and if something goes down or suffers from poor services too often, or even if perception starts to give people that impression, it's a problem.
Apple's building more data centers and hiring more internet services engineers, but this is absolutely a problem they have to fix and fast. iMessage and Game Center -- iCloud in general -- has to feel like power, like something that's almost taken for granted as always being there.
And when it does go down, it should be so rare as to be truly shocking. Not simply, "again?"
As always, let me know if iMessage is working for you, and if it isn't, when it starts working again.