Through the efforts of many - this has been achieved - at least in basic form.
The n900 detected two USB devices - though did not have drivers for them.

Egoshins post of earlier today. http://talk.maemo.org/showpost.php?p...&postcount=808

This is _not_ quite user-ready yet - but with the addition of http://talk.maemo.org/showpost.php?p...&postcount=803 - USB internal power - first got working by joerg http://talk.maemo.org/member.php?u=35572 it's almost there!

In a week or two - we should be able to (with the appropriate cable) plug in (low powered) memory sticks or keyboards, and have it work. Or high powered stuff through USB hubs.

This includes stuff that vastly expands the possibilities for the n900 - imagine a USB display adaptor showing a 'normal' desktop on any LCD.

However - this requires a fair bit of software work.

Getting to a state where this is user friendly requires work on understanding BME.
BME is the battery managment entity.

This controls many aspects of the phones power subsystem, and it looks like that to get powered hostmode at the moment, this needs to be killed/suspended. This means the phone won't charge when plugged in, and requires all sorts of hackery to pause BME.

Workarounds for this would be great, but it's going to take some effort.

To collaborate on this if you have kernel or reverse engineering skills: (we do not at this time need user perspectives, we know that plugging stuff into USB is cool. If you want to participate - thank those involved in the effort)

http://wiki.maemo.org/N900_Software_BME Reverse engineering of BME, and ways to get it to cooperate.
http://wiki.maemo.org/N900_Hardware_USB_Host USB host mode - kernel and userspace details.
http://wiki.maemo.org/N900_Hardware_USB An overview.

IRC - #maemo-hostmode-discussion on irc.freenode.com