Hello Steve,
The current "Select Inverse" option is available on your Right-mouse button context menu, under Select > Inverse Selection. In SpaceClaim 2009+ SP0 (the current release) this only works in 3D mode, and can be used to select faces, bodies, edges, etc.
The functionality will be added to Sketch Mode in the upcoming Service Pack 1, which should be released by the end of February.
Best Regards,
James Saunders