I think I've found some viable solutions for Japan usage (none completely ideal); if there's interest I can post more detail.

Basically my conclusions are:

• You can get voice connectivity by using a compatible sim card directly in the N900, or (probably) by using the N900's bluetooth headset profile to remote-control a nearby bluetooth-capable J-phone.
• You can get 3G data connectivity (wide coverage) by using a compatible sim card directly in the N900, or by using a portable battery-powered WiFi router such as the Cradlepoint PHS300, that allows you to plug in a 3G modem (verified as compatible with many Japanese models) and access it via WiFi.
• You can get WiFi data connectivity (narrow coverage) by looking for free hotspots (maps available online), or by subscribing to a paid WiFi hotspot service.

Regarding direct sim card compabitility with the N900, it seems:

• E-Mobile: Offer various voice-only, data-only, or voice+data plans. Probably won't work at all due to E-Mobile using the non-widespread UMTS Band IX (but possibly a future N900 firmware upgrade may support that). Cheap.
• Softbank: Offer various voice-only, data-only, or voice+data plans. Voice sim card will probably work. 3G data may work, but Softbank contractually prohibits using their sim cards in other devices on the cheap flat-rate plans; the more expensive business plans seem to allow sim-swapping. If you use a sim card in a non-approved device you may get hit with massive charges because Softbank appears very unfriendly to tethering or unapproved devices. Cheap.
• Willcom Core 3G: A data-only plan from Willcom. Verified to work in Android GDD phone (that runs on 900/2100 MHz, both supported by the N900) in Japan for 3G data connection. No voice on this plan. Slightly more expensive. Also the web page says the service may end in 2012.
• B-Mobile: Time-limited, prepaid, no-contract data plan. English documentation available. Per-minute charges are expensive, but if you don't use it much, it can be cheaper. The web page specifically says it's intended for surfing mail checking, so if you watch streaming video or use P2P on this plan you may get cut off. Interestingly, they also offer a sim-card-only prepaid plan aimed at Android developers.
• Docomo: Seem to offer voice-only or data-only plans. Not sure about details here. Slightly more expensive.
• AU: Doesn't seem to offer 3G data, not sure about details.
  

Regarding a WiFi data connection using the PHS300 as a WiFi router plus a USB data modem, then accessing the data connection from the N900 using Wifi, I found out the following:

• Willcom W-Value SELECT, a low-speed 64kbit PHS service. USB modem verified to work with PHS300: see 1, 2. Cheapest option: 980 yen/month, for max of 24 months (after that it reverts to standard price of 3700 yen/month). Willcom also has another similar offering where you buy their WiFi router instead of using the PHS300.
• Willcom Core 3G: verified to work with PHS300, requires using some initialization commands.
• E-Mobile: Many modems verified to work with PHS300. See PHS300 documentation.

Others may work, but you should check before buying. Note that the PHS300 does not run Linux, so you are dependent on the manufacturer to update their firmware to support newer USB data modems.

Regarding paid WiFi hotspot services, there are a couple of these available, but they only work in e.g. downtown Tokyo, and don't have nearly the coverage of 3G data. But depending on where you live/when you need data, WiFi may be a better option for you.

Personally, I think I'm going for Bluetooth headset profile for voice, and Willcom Core 3G for data. Continuing to investigate, though.