I wouldn't say I have personally noticed it being any quieter. As with any type of forums it relies on various factors to make it busy and there are always going to be busy & quiet times.
The main factor for keeping a site busy has to be it's members, and I guess it's core member base at that. However, there are times when those members are busy with their 'normal' lives, work, family, holidays etc which means for whatever reason they don't contribute as much as they usually/want to do. This last week I have been so busy with work & things happening at home that I haven't been able to spend as much time as I normally do on the site, so a lot of topics have passed me by.
The other thing that is needed are things to talk about. As Nick said there are only so many topics that can be discussed, so the site needs things to happen in the industry, or people to have questions/opinions etc that need to be answered/discussed/debated etc.
I don't think there is anything fundamentally wrong, and that maybe this week has just been one of those times when it is a bit quieter than normal. I guess Pete would know the figures on views/posts to the site to see if it has been any quieter than normal or if there has been a decline over a period.
Edit: just seen Pete's reply which seems to suggest the site isn't in decline[y][y]
Neil