It would be better to set the dimension ID in the config file rather than just grab the next free one.
Currently this clashes with Applied Energistics 2 mod which tries to register dimension ID 2 after Overworld Mirror

This should now be fixed

Well its the normal way to register dimensions. AE2 should change their static behaviour. But i'll provide a config option in the next release.

thanks, that's probably reasonable, pretty much any mod with extra dimensions lets you change it somehow (pack I'm putting together currently has 57 dimensions!)
However, I have managed to change it for AE2 - for some reason I missed it first time.

it's in the config file AppliedEnergistics2.cfg as:
I:storageDimensionID=2

change this number to something else, e.g.
I:storageDimensionID=4

Then Overworld Mirror can use ID 2 :-)