The Iwama Dojo - now officially called Ibaraki Shibu Dojo - takes uchi deshi also. Please refer to the dojo homepage: http://www13.big.or.jp/~aikikai/index.html
Isoyama Sensei is the Ibaraki Shibu Dojo Daiko (acting dojo cho). Inagaki Sensei is the General Manager. He also takes care of the website. You have the opportunity to train under these two Sensei, who both have had extensive training directly with O-Sensei, as well as people such as Hirosawa Sensei, who also has had a very close relationship with O-Sensei in his final years.
Nemoto Sensei, one of Saito Morihiro Sensei's closest Japanese students also has his own uchi deshi program in Iwama. You are welcome to contact me if you wish more information regarding these possibilities.
On my own homepage www.aiki-shuren-dojo.com
I have compiled information regarding being an uchi deshi. Although this information is based on my experience as uchi deshi under Saito Morihiro Sensei, and also with Nemoto Sensei, I believe it also will serve to give some advice for going to other places, too. There is quite a bit of information regarding etiquette which is good to know.
The information given by Shaun was very good. I would also like to point out that traditionally one is expected to be a deshi of one given Sensei. Shopping around between many Sensei is sometimes not taken too positively. As was mentioned in Shaun's text - feel the waters before speaking of your plans to move around.