What is the easiest way to login to my N900 from the PC while using it as a 3G modem? I normally use USB rather than bluetooth for the battery's sake.
I can ssh from the PC to the public IP address of the phone, but that is very slow - must be routing over the 3G network and dropping some packets.

I see the n900 shows as a usb0 ethernet device on my linux PC (as well as /dev/ttyACM0) when connected. Is it possible to use that usb0 device instead of PPP?

The easiest way I found was to download and use mobile hotspot.
Set up mobile hotspot using your 3G connection as default and start the app.
Start ssh on n900 then connect pc to n900 wireless point.
connect to n900 with winSCP program.
your n900 can still be connected to pc via usb throughout the ssh session so your n900 is still charging.
You can surf and ssh at the same time through wifi.
I have also noticed that my n900 can share the 3G connection with my pc, i can use it as a modem and use micro browser at the same time.

My earlier reply seems to have failed.
Thanks Mart.
I looked at mobile hotspot, but its a bit alpha. Did not work for me. Seems to need a special kernel that is not in the package dependencies. And no security, ad-hoc only, ...
Seems easier to keep switching between 3G and wifi, though neither is elegant.

It's worth giving mobile hotspot a try, you are right about a different kernel being required but no great drama here, it gives you more options to connect your n900 and not only for ssh sessions. I am not an expert in security but most connection types have security issues, As yet I have had no security issues when connecting via mobile hotspot.
Cheers
Mart

You can set up Usb networking here's the wiki.

http://wiki.maemo.org/N900_USB_networking

However I recommend backing up before you try this. When I tried this it messed up my Internet connections applet in the status menu.

Oh looking at your requirements you will still have a bit of work to do to get both connections working.

Thanks mrsellout, I should have found that.

Getting the USB link was as simple as "# ifconfig usb0 192.168.2.14" on my Ubuntu PC. I did not have to type any commands such as ifup on the N900, contrary to the wiki. It seems to be automatically configured when I plugged in the USB.

However it does not work if I have a PPP connection to the N900, either over USB or even Bluetooth.
On the plus side, it was easy to get usb0 working at the same time as wifi or 3G. I just had to remove the 'gateway' entry from /etc/network/interfaces to stop it changing the default route.

The solution is in http://wiki.maemo.org/N900_USB_router .
You need an enhanced kernel (Titan's kernel-power) to do NAT.
It seems to work for me using the power-kernel from testing. No need to enable devel now.
The only downside of this so far is that my PC gets a NAT-ed address, rather than the public IP address it would get by PPP before.
- posting this reply over USB networking :-)