What are the most notable cases of coins or wallet thefts? What are the most notable cases of coins or wallets being stolen from Bitcoin users or services? When did they occur, how many coins were stolen and has any of the thieves been identified?
In order of date:


13th June 2011, 25,000 BTC stolen from an early adopter "allinvain": A single user claims they were hacked and lost an enormous quantity of Bitcoins.
15th June 2011, 650 (?) BTC stolen from MtGox exchange: This attack probably made the news more than any other because the apparent market price plunged to $0.01/BTC, but withdrawal limits actually prevented a major loss. There was however substantial downtime and confusion amongst users while security issues were resolved. MtGox remains the largest exchange despite the issues, as at the time of writing.
31st July 2011, 27,000 BTC stolen from mybitcoin.com: This was a website that worked like an online wallet. After the website disappeared for several days, the owner reappeared claiming the site was hacked and is closing the site down, with a plan to refund 49% of each user's previous balance. There has been speculation that the owner of the site scammed the users.
1st August 2011, 17,000 BTC lost from Bitomat exchange: A surprisingly large Polish exchange was mistakenly (due to a technical misunderstanding) set up in an unstable hosting environment and lost its wallet file when the server crashed. The MtGox exchange took over Bitomat's business with an agreement to try to restore users Bitcoins.
1st March, 2012: More than 43,000 BTC were stolen from bitcoinica.com due to a Linode exploit, and another 3,000 from an individual programmer. The same exploit was used against Slush's Pool to steal 3094 BTC.
11th May, 2012: 18,547 BTC were stolen from bitcoinica.com. 


I'm not aware of anyone being caught for any of these thefts.