Possible Duplicate:
How many times an Android Market's application can be installed?
If I buy an app and after a few days I decide to wipe out the data on my phone and install a new ROM, will I have to buy that app again or can I download it for free since I had already bought it earlier? Can someone please confirm?
No, the market keeps track of your purchases.
Purchased apps can always be downloaded using the Android Market, assuming you are signed in with the account that you used to make the purchase.
No, your App purchases are tied to your Google account. This also allows you to install a purchased App on more than one device. For example I can install Apps that I bought on my smartphone and on my tablet.
As Gary mentioned, so long as you set up your phone with the same account(s) you had on it before you flashed the ROM, the apps will download and install for you. Your apps follow your account, not your phone.
You can also use an app like Titanium Backup to backup your app(s) + settings and then restore them after flashing a new ROM. This is my preferred method, as you don't have to spend a lot of time on the settings again. The free version lets you do one backup total at any time, which is good enough for a casual user, and there is a batch mode to let you back them all up at once (I don't recommend doing that with system apps across ROMs though as you may get strange results). It does require root, but if you're flashing a new ROM anyway that shouldn't be an issue.
