A WebmasterWorld thread has several different webmasters reporting seeing huge spikes in Google referrals recently and then those spikes dropping down.

The first post was on February 28th, but the spikes continue to happen on random days for several webmasters. Here is the first post:

Old time member, robzilla, found something similar:

It is hard to say if this is regarding a Google change or a change in the site or just a weird bleep. But it is interesting to see a nice set of diverse webmasters notice this and discuss it over the course of two weeks at WebmasterWorld.

Forum discussion at WebmasterWorld.