Putting aside my hilarious comment from yesterday, we did make an inquiry about this! The dev. team liked the idea, and know it's a popular one for many reasons (such as those listed in this thread), and they do want to implement it. However, items currently place a huge load on the server, especially the Market server, so this isn't something that can be implemented immediately. Once server-related issues are resolved, which plans are in motion to do, we can start working on implementing signatures, etc.
Sorry that it's not a technical breakdown of what the exact programming-related issue is, but hopefully we'll continue to get good ideas for future additions, regardless of any current technical hurdles (that are being addressed).
