If shared mirrors is still some time away, it would be useful to extend the Analysis - Compare Parts to calculate mirrors parts. This would allow you to mirror parts, then as the design progresses validate that the two related parts are staying in sync. Perhaps it could just be another drop down in the Part Alignment list - Auto-Aligh Mirrored
see also
http://communities.ptc.com/ideas/1241
http://communities.ptc.com/ideas/1171
