I managed to find a nice domain for a fairly fledgling site of mine that actually hasn't been parked by scumbag squatters. Given the upcoming move I'm thinking I'd take the opportunity to split the content across subdomains according to language, much like wikipedia for example:

current:
www.old-domain.com/en/subject  # English   
www.old-domain.com/subjecto    # Spanish (default so not locale in url)

proposed
en.new-domain.com/subject
es.new-domain.com/subjecto

The advantage of doing this is a fairly competitive keyword such that I may wish to put a copy of my application on a Spanish slice in order to gain a few serp's. Also pure vanity.

Google's webmaster tools allows me to move to the new domain and I can add the root domain and the subdomains but forward to only one. I'll 301 from the old domain appropriately but is there anything I should know about webmaster tools in this respect where effectively I'm moving to two addresses?

(Feel free to dissuade me from doing this if it's a bad idea in comments.)

I've now asked this same question on google's forums.