Your application purchases are tied to your Google Account and can be installed an unlimited amount of times on any device. So, for example, if you remove ‘My Favorite Game’ to save memory, you can reinstall it at a later date with no charge by simply visiting My Apps.
Please note that you can’t transfer paid apps between different Google Accounts.
For more information, please see http://market.android.com/support/bin/answer.py?hl=en&answer=113410