So I did a little experiment after logan23 asked me about this today...
There's this really awesome looking Bounty Hunter helmet that I put in IMB's guild vault, a red one that looks kind of like a cross between Boba/Jango Fetts helmet and the helmets the red Imperial Guards wear.
I took advantage of the new mailing system and used Silvana to send this helmet to Lynk.
When it arrived on the Republic side... the helmet changed... it was no longer that awesome looking Bounty Hunter helmet that it was on the Imperial side, it was a crappy looking bucket duck-face helmet no one uses cause they look completely lame.
I sent it back to Silvana and again, magically, it returned back to its original state.
So yes, while you can send gear across the faction line... you may not like the results as it comes out the other side... or perhaps things may turn out to look much cooler than you had previously thought.