Sure. In general, E-Mobile offers flat-rate data plans and recently have added voice plans. The flat rate data is 5000 yen per month with zero monthly fee for the voice (billed as needed).

Some links:

Here, see the first comment in the bottom that says "But EMobile has since constructed its own network towers and its devices run on 1700mhz (UMTS Band IX)."
http://www.akihabaranews.com/en/news...work.html/umts

Here, see some Japanese guys talking about differences between Band IV and Band IX and lamenting that Band IX isn't supported:
http://excite.co.jp/world/english/we...=JAEN&wb_dis=2

Here, see some poor guy trying to get an iPhone working on E-Mobile, but it doesn't work because the iPhone doesn't support 1700 MHz. Search for the comment, "EMobile in Japan is a pure 1700 Mhz network":
http://www.mobileinjapan.com/group/i...ne-in-emobiles

Here, another confirmation that E-Mobile uses Band IX:
http://androidcommunity.com/forums/a...p/t-19681.html

More comments about Band IX in E-Mobile; search for "Carrier e-Mobile in Japan also runs UMTS 1700, and has already released a handful of "UMTS 1700" WinMo handsets. However their band is specifically the UMTS band IX which is not compatible with T-mobile's UMTS band IV.":
http://www.wmexperts.com/articles/xp...tmobile_w.html

E-mobile's home page (in Japanese though) is, predictably, http://emobile.jp.

So... feasible to use an E-Mobile sim card (they call it "EM Chip" but it seems to be a regular sim, since someone managed to put it in an iPhone) in the N900?