Thanks for commenting and voting on this issue. We know that the ability to change the logo in the Confluence header is very important, especially to OnDemand customers who have recently been upgraded to the new header.
We have been working on this feature and it will make our upcoming release of Confluence (5.0). It's in the BETA of the release (which will be out in a couple of days) and I am hopeful we will be able to ship you this feature to OnDemand and install customers in around a month.
Thank you for your patience.