Daniel Jaikut asked why the default keychain in OS X keeps getting changed, which I’ve also noticed, as have a few other people. It seems to be related to XCode and usually manifests itself as a code signing error. If you look at the provisional profiles tab in XCode’s organizer window, you’ll see a warning about a missing certificate.
I found a very simple manual fix for it. Add the following script to XCode’s script menu:
#!/bin/sh /usr/bin/osascript - < <*** tell application "Keychain Scripting" set current keychain to keychain named "mike.keychain" end tell ***
If XCode complains about a missing certificate, run that script from the menu and you’ll be OK.