Hey Mike,
Could you confirm something for me. Is the data lost only in our extension or is it also really lost in mailchimp? We could change the way it works
I'm always happy when users propose meaningful fixes
Cheers,
Daniel

Many thanks for picking this up Daniel. The data is lost in MailChimp. I've tried updating my personal data from the CMC form and if I go into the MailChimp back end afterwards any fields that I left blank have been set to null.
We are a non-profit member organisation. Members can join on line and anyone (member or not) can sign up for the Newsletter here http://www.eghf.co.uk/index.php/newsletter
When someone signs up as a member we will also add their details on to the Newsletter list if it's not already there. We have hidden fields in MailChimp called "Member To" which holds the date their membership expires and also a Membership Number field. We use these to choose whether to send e-mails to everyone or only those who are current members. We also use it to send out membership renewal reminders. So there are fields visible to users and fields that are hidden.

If someone who'se already on the list uses the form (URL above) and leaves any of the fields blank then those fields are set to null in the MailChimp list.
If however we were to embed MailChimp's code the user is told the data is already on file and gets an update link. If he clicks it he gets an email with a personalised URL, if he clicks that all of the visible fields are displayed to him with his current data so he can check for accuracy and amend where necessary.
The loss of data does not apply to hidden fields nor does it apply to optional check box fields (the "Interested in" checkboxes on the CMC form). The data in those is retained however other data on the form such as the address fields is set to null.

Mike