OPM conducts all backgrounds for the military and civilian-government jobs. However, as stated above, any special agent or federal law enforcement positon will conduct their own background investigation with poly. While it is an incentive, it won't be the reason you get hired.
FBI isn't a very military-friendly orginization, either. In fact, I've read in some place that the FBI does not permit current special agents to be in any reserve military. Look towards ATF, USMS, DEA, also. People get so hung up on FBI. While the nature of the work will be somewhat different, the job-series is the same and you will be doing a lot of the same things. Often times, you will work alongside agents from multiple agencies on cases, anyways.
I wouldn't go that far, but I would say that they tend not to place that much weight on Veteran's Preference.
There is a HUGE push in the FBI to recruit those with software/network expertise. Typically, your average "most competitive" applicant will have a grad degree in a critical skill area, 2+ years of work experience, and military experience. But based on ( http://www.fbijobs.gov/CyberCareers/ ), the FBI is deeming those with a BS Computer Science/Electrical Engineering and 1 year of work experience as "most competitive".