Nice! I just added this to the services section of Prefixr.

The relevant part of the service is:

curl -sSd "css=$@" "http://prefixr.com/api/index.php"

Which is the same as the example for TextMate at prefixr.com/api/usage, but with $TM_SELECTED_TEXT replaced by $@.

At the moment there's no way to access the parameters from the Prefixr API (?). The output always uses multiline rules (vs inline rules) and tabs (vs spaces). And the input has to be full rule sets (like .box {border-radius:4px}), not individual declarations (like border-radius:4px).