There have been interruptions in Google Analytics in the past. Usually they're like this one where they announce that they're collecting the data, but there will be a brief delay in having it show up in the reports. I've also seen cases where there's a slight delay in getting data into a particular account with others working just fine.
But what's happening now is far worse. From my estimations there's a huge delay and no notice posted on the Google Analytics blog. Here's some evidence from a variety of sites...
A consumer health site:
The personal blog of a someone I know:
Another personal blog:
These aren't sites with insignificant traffic and they're about as broad a spectrum of sites as you get and they're all showing exactly the same trend - no data since 2 or 3am on the 29th, and substantial under reporting starting at the morning of the 28th for some sites and the evening of the 28th for other sites.
The last day shown in each of those graphs is yesterday - 7/29. Which means they were down all day yesterday, a substantial portion of 7/28, and so far today...
What bothers me the most is the lack of communication on the part of Google. Hopefully they're still collecting data and it's just a matter of time before it all shows up again in the reports.
UPDATE (1pm ET): Google has posted something on their blog acknowledging the problem and saying it's just a reporting delay... Let's hope.