On this past Tuesday, Intuit, the developer of QuickBooks for Mac, announced that the company will not offer support for QuickBooks 2009 and 2010 users who upgrade to Mac OS X 10.7 Lion.
If you are using QuickBooks 2009 or 2010 and plan on upgrading to Lion, you must upgrade to QuickBooks 2011.
If you do not plan on upgrading an earlier version of QuickBooks at this time, you will not be able to upgrade to the new version of the Mac OS Lion.
If you would like to upgrade to Lion but continue to use an earlier version of QuickBooks, you can can set up a separate Snow Leopard partition on your Mac to continue running QuickBooks 2009 or 2010. This option would require a reboot into the Snow Leopard partition each time you would like to run QuickBooks. (Not the best solution, but a solution nonetheless.)
A detailed explanation of the situation is available here.
One final note, it's possible to use the QuickBooks Online service to manage your books. In this case, you may upgrade to Lion and still work with a version of QuickBooks. You can find out information about QuickBooks Online here.