Prior to patch 1.2, you could NOT mail items to your cross faction alts. Now, however, that is not the case. You can mail your cross faction alt any item now you wish.
It is now a lot easier to exploit and profit off of price differences between the GTNs on each side.
On my server, for example, Mandalorion Iron sells for about 4k each higher on the Empire side than Republic. I buy all I can on the Republic side and resell it at 4k higher each on the Empire side.