All My Files is great for opening a Finder window and easily seeing recent documents. I found list view more useful with lots of PDFs and other docs with long and similar names. But list view will show an endless list going back as far as your oldest files. I wanted something else, in this case, only a list of files opened in the last month.
I created a new search and then made that 'All My Files.' After creating and saving the search:
Backup the file:
/System/Library/CoreServices/Finder.app/Contents/Resources/MyLibraries/myDocuments.cannedSearch/search.savedSearch
sudo cp ~/Library/Saved Searches/[your newly createdsearch].savedSearch /System/Library/CoreServices/Finder.app/Contents/Resources/MyLibraries/myDocuments.cannedSearch/search.savedSearch
[crarko adds: I admit I'm not sure how this is any better than just creating a new Smart Folder with the appropriate search criteria. But if you so want to modify All My Files instead here is a way to do it.]

