Here's one way, there may well be others.
I haven't used 2007 - I've done this in 2003 and assume you can do the same in 2007.
Make sure you are using a stacked bar chart type.
Turn off the data labels for your main series.
Use a dummy range to calculate the percentages you want to show.
Add this range as another data series.
Make sure it is in the correct sequence so that this new series sits on top of your existing series.
Set the labels to appear for this range.
Then change the format of this series to no fill, no border, so that it is effectively invisible, except for the % labels, which should be floating above the main bars.