Search keyword => tag

[cedivad]

This isn't a modification request, just because i think i will code it by myself, i would know if anyone have already done it; this is the idea: when your thread receive a visit from a search engine, the script must add the keyword to the thread's tag list.
I think it is easy to employement and very usefull.. if someone have already do it or have any suggestion about, please help. Thanks.