I have a Quicksilver like quick start program I cobbled together a few years ago, and I recently added a feature like this. It's basically a trivial macro expander, but it really does fill a serious need. It's only been a day or two, but I already have macros for weather, bookfinder, amazon, imdb, orbitz, google, apple support, ebay, wikipedia and the washington state liquor board, and I use them. The orbitz macro takes a city pair and two dates, so it uses a slightly different substitution ($1, $2, instead of $*).

What really helps is that many sites use POST forms, but doing the appropriate GET tends to work as well.

What limits this approach is that many sites require state, specifically cookies and session ids, to do searches. I have another program that handles this with a little scripting language and macros for keychain access, but I am not planning on going for that much power and complexity until I really need it.