Are querystring parameters secure in HTTPS (HTTP + SSL)?

Yes. The querystring is also encrypted with SSL. Nevertheless, as this article shows, it isn't a good idea to put sensitive information in the URL. For example:

URLs are stored in web server logs - typically the whole URL of each request is stored in a server log. This means that any sensitive data in the URL (e.g. a password) is being saved in clear text on the server

remember, SSL/TLS operates at the Transport Layer, so all the crypto goo happens under the application-layer HTTP stuff.

http://en.wikipedia.org/wiki/File:IP_stack_connections.svg

that's the long way of saying, "Yes!"