Take a look at the toArray() method for the Worksheet. There's a whole host of arguments to determine how you want the data returned, all defined in the API.
* @param mixed $nullValue Value returned in the array entry if a cell doesn't exist
* @param boolean $calculateFormulas Should formulas be calculated?
* @param boolean $formatData Should formatting be applied to cell values?
* @param boolean $returnCellRef False - Return a simple array of rows and columns indexed by number counting from zero
* True - Return rows and columns indexed by their actual row and column IDs
In the latest SVN code, there's also rangeToArray() and namedRangeToArray() methods.