Please talk me down on that:
say I get my n900, I'm happily roaming in the wild world, and I get at the office of a coworker or at a friend's home... and I need some charging of my precious...

Well I open my (geek?) bag, take the usb cradle and plug to his/her computer...

and - here starts my nightmare - my flash drives are mounted by his/her machine, and
1) vfat partitions get flooded by virii, files get encrypted by some obscure fellow who wants money by western union to send me a maybe password;
2) a trojan rsyncs everything and my personal documents are sold by mafia to the best bidder;
3) everything is thrashed, erased, and my precious blows away like any trivial iphone...

Now, am I paranoid? Can I avoid automounting? Should I bring the mains charger everywhere and avoid usb ports of computers as hell?

Thanks

buy another usb-cable and cut data wires from it?

No. You need the data connections, unfortunately. Nokia has this bad habit of requiring data to be sent while charging (should be an optional feature). Someone might create something similar to the USB OTG applet to manually override automounting. You might also consider something like the Lenmar powerpack mini. Plug the N900 into the tiny lithium battery and then a cable from that to the computer. That or pack a USB=>AC adapter.

I'm sure something will be created to avoid this problem. The reason it doesn't exist for the N8X0 is that one cannot charge via USB (from the mini/micro-usb port; 1mm=>usb cables work) but with the N900 the problem will present itself and someone will provide a fix (assuming Nokia doesn't).

they're small

I wouldn't worry about this. Released ke-recv sources reveal that there's probably an interactive selection of mode. That means, no mass storage provided unless explictly told to.

On the existing tablets, keeping an open file on a volume prevents it from being exported to be mounted (to my frequent irritation).

Plugging it in causes it to do a umount, which can fail. It would be easy to create an applet to prevent it.

Yeah, I was going to suggest exactly something like that. debernardis, use a usb charging brick like what ysss suggested. If you can't use a brick, then get a portable battery powered battery charger. We talked a lot about them in this thread here. I personally have the mp1550 and love it.

I heard there are special pills that can help.

Thanks all... I know, I have one of those bricks, and one for my car but... a usb cable is much smaller especially a retractable one.

@Linuxeventually, what happens if you cut the data wires? Does it mean that the mains charger has those data wires connected in order to charge devices like the n900?

^^^^^^^^^^^^