How to get Excel XML to keep leading zeros in strings?

I am trying to generate an XML file through Ruby:

<ss:Cell>
    <ss:Data ss:Type="String">0000023</ss:Data>
</ss:Cell>

However my invoice numbers all display in Excel with a green (and ugly) comment triangle. When I then click the cell, the leading zeros are stripped and the comment disappears. This is not what I want since the zeros are part of the invoice numbers. How can I prevent Excel from doing that?

The strings are coming from a MySQL database type varchar255 field by the way. So they are strings from the very begiing and shouldn't be considered numbers by Excel anyway.