Prior to Windows Live ID you were able to change the primary email address on your Passport account (assuming your primary email address was a non-Hotmail address). This feature was missing from the new Windows Live ID system and once you migrated from Passport to Windows Live ID there was no going back.
Some people (like me) who changed jobs were stuck with either creating a new Windows Live ID (and chucking everything associated with the old one) or continuing to use the old account with an old email address. The later option was a bit scary if, like me, your old email was being forwarded to someone else; allowing them to retrieve your password and access your account.
On a whim I checked today and it looks like the Live team has rolled out the ability to change your email address. Simply go to https://accountservices.passport.net/?lc=1033 and select a new one (either a Hotmail or another external email address).
Thanks to whomever pushed this one though.