Leonard0 - 9:17 pm on Aug 2, 2012 (gmt 0)

Isn't that what it means? That an ad wasn't available?

Yep, it was discussed here almost a year ago when it first appeared in the reports.

The OP doesn't mention which report is showing Unmatched Ad Requests.

I see it in Targeting Types and in Bid Types (in the bottom row).
You can see which sites are generating unmatched ads by clicking the Dimension button and selecting Sites.

If your channels for non-Adlink blocks are showing 100% coverage (or close to it) there shouldn't a problem.