When the Apple store "goes dark" it's usually because they're adding new products.
However it's gone dark this morning and as far as I know no new products were announced... or maybe I missed it..
Or, it could simply be that they've run into some technical issue that is affecting their site
Update: It seems it was a technical glitch or maintenance
In the last MacWorld keynote they said that iLife 09 would be shipping end of the month. Maybe it has something to do with that or other products announced then but not released until now.