Given all the recent uncertainly surrounding spectrum auctions, like many others I have been following articles on the issue as best I can. In an Ars Technica article over the weekend comparing the Verizon spectrum buys to the AT&T/T-Mobile merger, a commenter ("doormat") noted he had quickly created a mapping tool to display where the various cell phone companies and other major groups held spectrum. He compared it to the FCC's spectrum dashboard but I think it's better than the FCC's work.

It is rough around the edges, but I found it to be very useful nonetheless. Have a look.