Is there any way to measure relative occurrences of wallet theft over time? Soon after Bitcoins became valuable enough to be worth stealing, I started seeing news about viruses/malware that steal Bitcoin wallets. Presumably, wallet encryption has reduced the level of this, but is there any way of measuring it? 

Given that there is probably growth in the number of users, I guess you'd have to compare it with that figure to get a comparative figure over time for the chance of having a wallet stolen. Is that figure measurable?
The only way you can measure something like that, is if users would first of all know if their wallets get stolen, and second of all, if they reported that problem. Besides that there is no real way to know if a wallet actually got stolen, nor are there any means of verifying that it did.

On the other hand, if one was to steal a wallet from some big Bitcoin website (say, a pool, an exchange, or an eWallet), one probably could notice a change of pattern of transactions, and we might hear some official statements from website owners about the theft. In this case though, it would probably be more of "quality over quantity" theft, as in we wouldn't see many cases like this appear.