I have used Google Doc viewer (embedded version) in another project and it is far more limited than the Doc viewer that is part of GMail, which may result in hitting a brick wall development wise.

Edit: Are you using the proper embeded version or a hack of Google Docs - its prompting me to login from an iframe.. this is a definite! problem. Email me for advice neurofog <at> gmail <dot> com if you want the details.