Not sure. HBO has until sometime in June to decide. I'm pretty sure the last episode matched Deadwood in ratings, so I'm betting HBO will give it another go. Hell, The Wire is getting a fourth season, and even though I'm sure that's cheaper, it had much worse ratings. With Six Feet Under and The Sopranos ending soon, they might want to keep some solid shows around as well in case their upcoming new ones fail to connect.
It appears that HBO will be experimenting with showing its original series on nights other than Monday, so that could be a good sign. They may bring the shows back and just show them on a more sparse night, and not opposite Desperate Housewives on Sundays. The last season of Six Feet Under will be on Monday nights, so I'm guessing that they're hoping to see whether that can be a viable night.
So, personally, I'm going to be rather surprised if it doesn't return.