Android Market Refunds
Google allows you to trial any paid-for commercial app from the Android Market for free, for 24 hours. You buy the app using the google checkout process. But if you decide you don't want to keep the app, simply uninstall it and you will get a refund, as long as you do this within 24 hours.
No Refunds For You
But, I have found out, to my expense, that you lose the refund option if you buy the app again within the same 24 hour period. Don't ask why I was buying the same app twice. :-) The only way to get a refund in this case is to log in to your Google Checkout account, and email the software developer grovelling nicely.
Don't buy an app twice and expect to be able to get a refund!