From what I know, it will be sometime between now and a few weeks, and it varies between people. SP1 has reportedly broken compatibility with quite a few NICs, so if you own one of the affected NICs, you will have to wait. For the ones that are not affected, look for a download in Windows Update coming soon. (around a week or 2)
As I said, SOME people are getting it today. Also, not all languages are supported. Anyway, I've been running SP1 RTM for a while on my laptop, and didn't notice any difference really, though then again, my laptop is rather old.