HTTPS, aka SSL, only protects data in transit. Google switching automatically from HTTP to HTTPS means that the search query from your browser to Google's server gets encrypted. But on Google's server, the SSL tunnel ends, and it gets decrypted. This will prevent spying only under the two following conditions:
- The spies were indeed looking at traffic between the client machine and Google's server (as opposed to simply requesting from Google themselves a copy of the data).
- The user actually notices that Google redirects to the HTTPS URL, and "takes offence" if the redirect does not happen.
The second case is against active spies who would do a Man-in-the-Middle attack, which is highly doable on HTTP (no SSL). Since the redirect-to-HTTPS occurs at the HTTP level, before (of course) any SSL actually occurs, then the MitM can certainly block it and maintain the illusion of a non-SSL Google server.
In any case, for a big US governmental agency like the NSA, enlisting the help of some people at Google's, to get a copy of the query data, would probably be vastly easier and more efficient than the grudging work of spying on network lines. Google, being a US-based company, would comply or be made to comply for a (small) fraction of the cost implied by generalized spying. Retrieving query data from Google themselves would also work for queries which are inconvenient to spy upon, e.g. for a connection from a non-US country to a non-US-based Google server.
Therefore, I find it implausible that enabling HTTPS would really be relevant to purported spying from NSA. "HTTPS for everybody" is more a marketing / public relations move than an actual security improvement, in the context of Google and assuming that the "enemy" is the NSA.