I began to notice this last year. Roberts is a great first-half player. Check his stats the last two years and compare pre all-star and post all-star. The difference is dramatic. SB is a perfect example. In '04 he had 29 total, 22 before the break. This year he had 27, 18 in the first half and 9 in the second.
You do have to respect the doubles he hits, setting a new Oriole record in '04 with 50 and collecting 45 this year, 22 in the first half and 23 in the second.
I play in primarily auction leagues so it is difficult for me to correlate auction value to draft position, but I would estimate he will sell in our auction next spring for close to $20 which will be about what Kent will sell for. Soriano will go for $30+ but that gets dampend a little because most the leagues I play in use OBP instead of BA.
Final thought, pick up Roberts in your draft and ride him until July, then trade him for Tadahito Iguchi or Chase Utley. You could have got about any player you wanted for Roberts in July this season.