Hi Shaun,
Reading your thread, I´m not quite sure if you had SID imagens Inserted ou Connected to (via FDO).
But, apart from this, I think you should try installing Raster Design Object Enabler for Map 2008 (in your case).
If you don´t know about this, Object Enabler is free, and can be downloaded from Autodesk´s website. It will load the "extra" image file types that are now missing - including MrSID.
Cheers
Clara