This is so I don't forget, it took me a while to not find the answer via google, guess the solution successfully, and then read the solution by chance while looking at something else.
I use googlemail (actually, google apps but let's not get pedantic).
I have a friend with several email addresses, (work, other, home, other, other).
Recently she asked for emails not to go her work address (unless they're really urgent).
When I use gmail to write an email just to her, it's easy to choose which address it's going too (and I know roughly which address is most appropriate at the time), but I have a contact group set up with her in it and those emails were always going to her work address.
There was no option when creating the group to choose which of her email addresses should be used by default. The address being used (work) was also the address that came up first when composing a message and including her specifically.
I guessed that this apparent default email address was the first contact address I'd entered for her and when I checked it was showing on the top of the gmail list of addresses for her contact record. I replaced the top of list email address with the prefered 'other' address, added the work address to the bottom of the contact list. Created a new email and using the contact group and gmail used the 'other' address' instead of her work address.
I later read 'googlemail defaults to using the first email address of a contact....'