As chance would have it, I played poker tonight with a guy who has been a mortgage broker for 20 years, and I put your question to him. He stated that as long as the new house you're buying is equal or higher value, it is not a problem at all, UNLESS there is something specific in your contract to the contrary (which he said is rare).
What will happen is your mortgage will be reviewed, but unless you are trading down in terms of value, you're golden... and apparently as long as you keep trading up there is no issue, though since it is "subject to review" I'd imagine if you made a franchise out of doing it, buying a new house and moving every year, it'd be a problem.