Main difference is the iM-790 has a new, upgraded driver which is a little smoother and cleaner. The iM-590 has slightly more forward (Grado-esque) upper-mids.
Those sound pretty exciting, especially with your reputation for very good sound quality, high build quality and comfort.
How would you compare the two bass outputs of the iM-790 to the iM-590?
Will the iM-795 also be compatible with Blackberries, or will there be an iP-HS7 version?
Finally, will the 7-series use the 'fused double flange' tips like the iM-590?