I do know of people who have done what you’re proposing and have not had issues with entry into the country. However, there is always the slightest possibility that you could run into an immigration officer who will give you a hard time. If you want to be on the safe side, you might consider booking a return flight with a company that allows free cancellation within a certain time-frame. Once you enter the country, simply cancel your return flight and your money should be refunded. I believe Orbitz gives you a 24-hour cancellation period and American Airlines used to give you two days to cancel, though I haven’t checked lately to see if that’s still the case.