I've just got mine back after taking it to Carphone Warehouse, and it was about 3 weeks, unfortunately for me I didn't get a new one, but the USB port was only a bit loose, it hadn't come out completely. And they had given it a full reflash.

I used the terminal to move they files to my SD card quickly, fortunately I didn't have too much on the internal storage so I could move most of it in a single command, "mv /home/user/MyDocs/* /media/mmc1/" that will get most of the files (except the hidden dotfiles*), and Nokia in their infinite wisdom decided that some of the directories should be hidden with their file manager doing a translation so the user can see them, so you'll have to move those separately, so that's "mv /home/user/MyDocs/.documents /media/mmc1/" etc, for each one you want, don't forget you can use "tab" to autocomplete filenames. You can also use tar to save a copy of your home directory in case there are any settings there that don't get backed up by the backup program "tar czpf /media/mmc1/N900home.tar.gz /home/user/". If you don't want to use the command line to move your files there are other file manager programs you can install, I'm sure one or two of them will work better than the default one for moving a selection of files you want to keep.

To erase data fast, you could first use the "Clear device" option in the men in Settings, then you could just delete your home directory if you want to be sure nothing is missed. As root, first unmount MyDocs or this might not work properly "umount /home/user/MyDocs", then as root delete your home directory "cd / ; rm -rf /home/user", I'm not 100% sure it'll let you delete this directory while you are still running as a user, but it should still delete all the files in it anyway, then just in case you need the N900 to still be bootable, you might want to recreate it "mkdir /home/user" and make it usable "chown user:users /home/user".

*In Linux if you want a file to be hidden you prefix the name with a dot, and these files are sometimes referred to as dotfiles.