Edit: As far as I know this is a very standard clause in tech company employment contracts, and is perhaps the legal default even in the absence of such a clause. For example, see "Employed to invent" under http://www.nolo.com/legal-encyclopedia/pre-invention-assignm... .
California is an exception to the above: here employees may retain the rights to IP they create on their own time, not using resources of their employer (including company laptop), and, importantly, not in the same line of business as their employer. In the tech field, the "same line of business" caveat can be killer.