I got this beta timing question a few days back. I'm sure you've heard this before, but we don't announce beta/release dates mostly because we don't know exactly when the development/testing stars will line up.
While I can't tell you when the beta will be, I can tell you that we will be releasing a non-beta-quality build in a matter of weeks. This build won't be for everyone - it will have very limited testing, it may/will have significant bugs, it probably won't install/side-by-side/uninstall cleanly, etc.. But for someone with a spare machine and the patience/time, it will be an opportunity to see Whidbey's progress since last fall. We'll distribute this to alpha customers, mvps, pdc attendees, msdn subscribers, and attendees at some events in the near future. In the long run we want to make these “community” builds available broadly/publicly, but we don't have those logistics worked out yet and so we're starting with what we know we can do. I've mentioned before that we're going to do more frequent drops - this is the first one. We will also continue to do less-frequent beta-quality drops - with full testing, clean install/uninstall scenarios, etc.. -which will be intended for all customers.
More details to come in the near future.