SE have just been in touch on this - there's no DRM. Will update with full story shortly.
Purchase or use of this item is subject to the PlayStation®Network Terms of Service and User Agreement and this item's use restrictions. This item has been sublicensed to you by Sony Computer Entertainment America. One-time license fee for downloads to up to 2 console systems that are associated with the purchasing account. No more than one activated PS3 system within a 24 hour period. Content may not be used by any other account. You must sign in to PSN each time you start the game.
Tomb Raider is the new Final Fight Double Impact. Some people might think this is no issue, but remember that PSN does have frequent maintenance periods. You won't be able to play the game when PSN goes down, even in single player. If it's like Final Fight, it only requires Internet connection when you launch the game. Most likely to combat gameshare, but seems very inconvenient for legitimate users.
At least this time, they made note of it in the terms on the product page. But I know many people (almost everyone) don't read it, so I thought you guys should know about it before opting for a digital version of the game.
Aren't you able to sign into PSN most of the time during maintenance?
Actually this is true, but you have sign into the service recently before maintenance for you to sign on. It depends on the type of maintenance though