One way to mitigate this would be to allow users to limit search to who they following, or some circle around them. This might even be the basis of a business

One way to mitigate this would be to allow users to limit search to who they following, or some circle around them. This might even be the basis of a business model, if Twitter gets its micropayments story worked out: let folks who are good at finding good content charge other users to follow them (something small, 10c a month or whatever). People might pay if they felt like they were avoiding spam and getting good, searchable content.