Clicking on a document library link to an Excel document resulted in the following error:
Unable to process the request. Wait a few minutes and try performing this operation again.
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Edit the serverfilesExcelServer.xml file at the following location:
[Program Files]\Common Files\Microsoft Shared\Web Server Extensions\14\TEMPLATE\XML\
Comment out any lines with the file extension of xlsx or xls
Example:

<!– Copyright (c) Microsoft Corporation. All rights reserved. –>

<!–<Mapping FileExtension="xlsx" RedirectUrlTemplate="/_layouts/xlviewer.aspx?id=|0" NoGetRedirect="TRUE" CreateRedirectUrlTemplate="/_layouts/xlviewer.aspx?new=1"/> –>

Perform an IISRESET and you will now be presented with the tried and true standard “Open Document” dialog
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The Solution 2
EDIT: Updated solution post service pack 1               (checkout this link for details)
  1. Navigate: Site Actions –> Site Settings
  2. Click the Site Collections Features link located under the Site Collection Administration section of the page
  3. Locate and active the Open Documents in Client Applications by Default