According to a screen shot one user posted on Photosynth's customer service site, Microsoft suspended uploading "until the load dies down a little."

The topics at the site also suggest some people have been struggling to use the service. A sample:

"Stuck at 'experiencing a network problem'"

"Every time I start the installer, my pc freezes totally!"
"What is error A0000004?"

I've asked a spokeswoman for any info on what's going on and the size of the initial demand that seems to have knocked Photosynth down, at least partially. I'll update this post as appropriate.

Updated, 11:51 a.m.: The spokeswoman said via email, "They are absolutely overwhelmed with traffic and are adding capacity as quickly as they can to get the service back up and running."

Updated, 3:20 p.m.: Here's more from the Photosynth team, in a blog post explaining what happened:

"We have been abolsutely overwhlemed [sic] by demand, and have turned Photosynth.com into a special static/read-only mode for the moment. The team is hard at work adding capacity and getting the full site back online. We've been under incredible demand since we released just over 12 hours ago. With everyone waking up around the world traffic has been on a steady ramp up since that release and has far exceeded even our most optimistic expectations.

"Getting ready for the launch we did massive amounts of performance testing, built capacity model after capacity model, and yet with all of that, you threw so much traffic our way that we need to add more capacity. We are adding that extra horsepower right now and should be back up shortly."

Update, Friday morning: Photosynth was back online by late afternoon Thursday. It started accepting new synths again that evening.