As others have mentioned, Aperture 3 supports brushing in and out of adjustments on selected areas of the photo. As with a most image repair, you have to try a few things to see which technique works best for the photo in hand, but some adjustments to try:

• The Highlights/Shadows adjustment may bring back some detail, or can be used to reduce brightness of the highlights.
• If you shot RAW, the Recovery slider under Exposure may also be able to bring back some detail.
• Again, if you shot RAW, try reducing the Boost slider in the RAW settings (usually right at the top of the adjustments panel).
• The Burn tool, used carefully, may help you to lessen the contrast of the blown out areas.

When brushing in any adjustments there are a couple of finer settings which may help:

• Detect Edges should enable you to brush up to an edge of high contrast and not have the brush 'paint over the line'.
• If you click the little arrow top right of the brush HUD panel, there's an option to select the tone range the brush should apply to. In this case, you'd want to select Highlights. This will also stop you accidentally applying the adjustment to the wrong areas.
• The brush HUD panel has options to brush in and out, but there's also a feather option. Once you're done with brushing in, use the feather to blend the edges of the adjustment to make it less noticeable.

As Aperture is non-destructive, you have nothing to lose in just trying stuff out. You can always undo or revert.