This is the situation on the next day after the quake struck. 3/12 (Sat) around noon, according to my friend Asako Itagaki at WirelessWire News

NTT DoCoMo

FOMA (3G) voice service is occasionally interrupted due to congestion. There are cases that users get "no service" on their phone.
FOMA voice and data cannot be used in the areas with no power.
Calls to Tohoku and Kanto area are partially blocked to avoid congestion after 3:07pm on 3/11.

au (KDDI)

au voice service is occasionally interrupted due to congestion.
Mobile e-mail notification does not work in some parts of following prefectures: Hokkaido, Aomori, Iwate, Akita, Miyagi, Yamagata, Fukushima, Niigata, Igaragi, Gunma, Tochigi, Chiba, Tokyo, Saitama, Kanagawa, Yamanashi

Softbank

Softbank voice service is occasionally interrupted due to congestion.
Calls to the following prefectures are partially blocked to avoid congestion after 8:00am on 3/12: Aomori, Iwate, Akita, Miyagi, Fukushima, Yamagata, Ibaragi, Gunma, Tochigi, Chiba, Tokyo, Saitama, Kanagawa, Yamanashi, Shizuoka

E-mobile

Unstable transmission in the Eastern Japan, in some parts of the following prefectures: Aomori, Iwate, Miyagi, Akita, Yamagata, Fukushima, Ibaragi, Tochigi, Gunma

Wilcom

Unstable transmission due to the base station problems caused by power outages in Kanto, Tohoku and Tokai area

Michi