I have 7.5 under with approximately 5 players still needed: this includes rollover from 2011 (.5), credits likely to be used (1.5), RFA/EFA tenders, and draft picks (4.5). This cap number being relevant to the final 53.
http://steelersspreadsheet.wordpress.com/
Steelers Depot #'s include 1.6 additional cap dollars from Cowboys/Redskins, which I believe per PFT is already factored into the cap base of 120.6. In addition, his number uses the current highest 51 salaries.