Here is the first clue that Apple is ending support for Tiger. An update to Safari that takes it from 4.0.5 to 4.1.0 is probably the minimum they can do to support HTML 5 and other updates, but the leap to Safari 5.0.0 is reserved for Leopard and Snow Leopard.
There is little to say on the Apple site about what features are new in this version, but it's no secret that Apple wants to make sure they fully support all alternatives to Flash without abandoning it completely.
I'm downloading it now, and I will probably install it and start using it immediately, but not before making a clone backup that will let me revert to 4.0.5 if it turns out to be a total toad. Remember, unlike Firefox, it is impossible to remove all parts of Safari and downgrade to a previous version. If you don't want to risk it, wait a few weeks and see what happens with other users.
Oh, and in other news, Apple released a new iPhone. Looks pretty nice. 3GS users can get the new OS and will not be denied any software features of the new one. If you have been waiting on the phone, remember that the unlimited data plan is history but unless you plan on subscribing to Netflix for iPhone, you probably won't need that much bandwidth. ATT claims only 2% of their users burn through that much data, but they're probably exaggerating and it's closer to 20%. They have finally decided to sell us tethering capability (using the phone as a modem for your laptop) but the price they want seems hardly worth it.
I'll let you know immediately if Safari turns out to be a turkey, and after a couple of weeks if I have no trouble with it.