RIM’s attempt to rapidly clear stock of the BlackBerry PlayBook mounted on Thursday with a new promo for business buyers. Companies that buy BlackBerry Enterprise Server 5 before the end of 2011 get a free 16GB version of the tablet. The deal is retroactive to October 12 and still includes $400 off the cost of a pro install on a given company’s behalf.

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The giveaway isn’t as wide-ranging as an earlier two-for-one deal but is another aggressive step in trying to court companies and organizations. Despite touting early corporate deals located mostly in its home country of Canada, RIM has had trouble getting significant interest from large-scale buyers that have more often sided with Apple. About 92 percent of the Fortune 500 is testing or actively using iPads.

The adoption problem has commonly been attributed to RIM’s decision to rush the PlayBook to market without native e-mail, calendars, or contacts. On-device support through BlackBerry Bridge so far hasn’t been persuasive, as it requires that every user have a BlackBerry at a time when enterprises are increasingly open to using iPhones and Android phones.

Price drops to $199 across North America, along with the corporate deals, signal that RIM may still be having trouble clearing stock even after an earlier sale at $299. The company had to cut its shipments in the summer by more than half and is unofficially believed to have had plans to ship many more until low demand forced otherwise.