No leapseconds recently, so this thread is probably not your problem.  [Maybe fuzzter's query should be a separate thread?]

My Bitcoin 0.7.2 client began spending its time at 100% CPU busy a couple of weeks ago.  I was forced to shutdown my client.  Whatever is causing this is effectively a form of DOS, in my opinion.

My client has contained the vanity address for "correct horse battery staple" for many months.  [It's been interesting to watch tiny deposits appear and disappear from the account.]  I suspect the problem is related to the recent block-chain spam involving this vanity address.  Unfortunately, if the standard client provides a method of completely and totally expunging an address from the wallet, I haven't found it.  (I'm aware of a third-party tool to do the job.)  This seems like a pretty serious limitation (and DOS vulnerability) of the reference-standard client.

Coincidentally, at about the same time, my client has been displaying a "nag" message saying "Action required: see http://bitcoin.org/may15.html for more information".  I've already taken the other mitigation steps, but my client continues to be nag me.]

It's probably the block-chain spam causing the 100% CPU?  Maybe it's the nag message causing the 100% CPU?  Or maybe it's some combination of these factors.

BTW, after having had my wallet offline for a little over a week, my client consumed 318 minutes of a modern i7's CPU time over the last ~5.5 hours.  The wallet is now synchronized, but every time I blink, it goes back to 100% CPU busy for minutes at a time.  I can't imagine what the client might be doing that could *possibly* take 318 minutes of a modern CPU's time!