For those who don't follow the minors closely...
B.J. Upton is "the" icon of the minor leagues right now, a sure shot long-term star who is every bit major league ready in most areas except defense and attitude... where he basically sucks, leaving him in the minors so long. But he can't be held back any longer with any rational, so expect him up soon.
Big bonus is for keeper league owners, as Upton long-term projects into a superstar... a faster Julio Lugo at worst, and at best compared to Jeter. But just as is always the rule about rookies, he will have his struggles and issues when he's called up... with one exception.
The man is FAST. There are some who think he had more footspeed than Joey Gathright, who most consider the fastest base-runner in baseball right now (ohh, if only he could hit...). And he'll play for a team (TB) that LOVES to let fast guys run. Upton will take some time to develop 15-20 HR power, and may only hit a couple this year, plus his BA will be nothing to brag about. But he's pretty much a lock to steal 12-15 bases in the final two months of the season. If you need SBs, and there's no one intriguing (read: Dave Roberts) on the waiver wire, Upton's probably your best chance at a sure-fire double digit SB guy in the second half. If you lead your league in steals, or badly need power, you might let someone else jump on the bandwagon.
As for position eligibility, he's generally been a SS and qualifies as such in most leagues. However, the Devil Rays moved him to 3B a couple weeks ago, with the thought that it will hurry him to the big leagues (he looks plain ugly at times defensively at SS). And, unless Lugo is traded and they don't get a SS back, Upton will probably play 3B in the big leagues daily... meaning as soon as he qualifies there in your league, he'll be 3B-eligible as well. However, unless you need nothing but steals, Upton shouldn't be a regular fantasy starter at 3B. Good potential option as a SS or MI though, and great option off the bench to help with occasional steal as a fill-in.
Basically, if you need steals, take a chance on him. Otherwise, let him pass. And if you're in a keeper league, he's gone already.
Hope I helped.