In addition to trading for Rob Sims from Seattle, the Lions also traded away defensive end Robert Henderson to the Seahawks. The Lions received a seventh-rounder for Henderson, who as far as I can tell was merely a practice squad player last year. He was signed to the active roster right after the end of the 2009 season, but it looks like the Lions just picked up an extra draft pick for someone who was only on the practice squad last season.
UPDATE: Here are the picks the Lions now have, updated to reflect the latest trade:
Round 1 - #2 (2)
Round 2 - #2 (34)
Round 3 - #2 (66)
Round 4 - #2 (100)
Round 7 - #6 from Seattle (213)
Round 7 - #7 from Cleveland (214)
Round 7 - #48 (255)