Why can't I associate files with Excel after upgrading to 2010?

Found the problem while digging around in my registry.

HKEY_CLASSES_ROOT\Applications\EXCEL.EXE\shell\open\command 

was still pointing at the location of Excel 2007. Apparently the upgrade didn't update this?

Changing this to point at the location of Excel 2010 made it so that Excel would show up in the Open With dialog after I browsed to the EXE, so I could associate the file type and now it correctly opens and auto-formats!

In Windows Explorer choose, Tools | Folder options. Next, select the File Types tab. Click the New button. Enter 'TSV' in the File Extension box, then OK. Next, click the Change... button, select the 'Select the program from a list' radio button, then click OK. When the dialog options, scroll to find Microsoft Excel, select it, then click OK. Finally, close the Folder Options dialog.

I tried the prior post but it didn't work, probably because HKCR\Applications\EXCEL.EXE\shell\open\command was previously empty in my registry.

But it inspired me to search the Registry for the field values containing the directory Office12 and replace all occurences with Office14 (except when in an Office12 directory) and that worked for me.