My Bitcoin daemon has started taking up 100% CPU as mentioned here
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!