Yes, you can use the $http_user_agent variable to check against it
set $webkit_rewrite do_not_perform;
## http_user_agent for webkit ##
if ($http_user_agent ~* WebKit) {
set $webkit_rewrite perform;
}
## redirect to webkit.example.com ##
if ($webkit_rewrite = perform) {
rewrite ^ http://m.example.com$request_uri? redirect;
break;
}