Using SharePoint 2010 enterprise edition. 32 bit Project 2007 is installed on my machine with 32 bit MS Office. Have contribute access to the library.
The "Default open behavior for browser-enabled documents: "at Library Settings >> Advanced Settings is set to "Open in the client application". “SharePoint OpenDocuments Class” add-on is enabled in IE 8 was enabled via the IE Tools -> Manage add-ons -> Microsoft Corporation section .
When we click on the .mpp file in SharePoint, it prompts for check-out or read-only ALL OK
When clicking on a link in email, the .mpp files opens in MS Project but no option to check-out the file for edit purposes!!
With other MS Office documents i.e. .docx, it does give an option to "Check out" the document.
listed below are the screen shot from MS Project (no option to checkout) and MS Word (with option to check-out)