NM из свежего Сизифа?

Да, конечно.

Что в /var/log/messages при этом?

При подключении USB->Eth адаптера с подключеным кабелем в /var/log/messages имеем:

Jun  7 20:44:30 roman kernel: [ 3010.128087] usb 5-1: new high speed USB device using ehci_hcd and address 3
Jun  7 20:44:30 roman kernel: [ 3010.274293] usb 5-1: New USB device found, idVendor=2001, idProduct=3c05
Jun  7 20:44:30 roman kernel: [ 3010.274303] usb 5-1: New USB device strings: Mfr=1, Product=2, SerialNumber=3
Jun  7 20:44:30 roman kernel: [ 3010.274312] usb 5-1: Product: DUB-E100
Jun  7 20:44:30 roman kernel: [ 3010.274318] usb 5-1: Manufacturer: D-Link Corporation
Jun  7 20:44:30 roman kernel: [ 3010.274324] usb 5-1: SerialNumber: 000001
Jun  7 20:44:31 roman kernel: [ 3010.582605] ASIX USB Ethernet Adapter:v4.1.0 10:44:16 Jun  6 2011
Jun  7 20:44:31 roman kernel: [ 3010.582610] <6>    http://www.asix.com.tw
Jun  7 20:44:31 roman kernel: [ 3010.582621] eth%%d: status ep1in, 8 bytes period 11
Jun  7 20:44:31 roman kernel: [ 3010.583400] eth1: register 'asix' at usb-0000:00:12.2-1, D-Link DUB-E100 USB 2.0 Fast Ethernet Adapter, 00:80
:c8:3b:cc:cb
Jun  7 20:44:31 roman kernel: [ 3010.583473] usbcore: registered new interface driver asix
Jun  7 20:44:31 roman NetworkManager[4574]:    etcnet-alt: read connection: Failed to open file '/etc/net/ifaces/eth1/ipv4address': No such fi
le or directory
Jun  7 20:44:31 roman NetworkManager[4574]: <error> [1307468671.60826] [nm-device-ethernet.c:750] real_update_permanent_hw_address(): (eth1): 
unable to read permanent MAC address (error 0)
Jun  7 20:44:31 roman NetworkManager[4574]: <info> (eth1): carrier is OFF
Jun  7 20:44:31 roman NetworkManager[4574]: <info> (eth1): new Ethernet device (driver: 'asix' ifindex: 4)
Jun  7 20:44:31 roman NetworkManager[4574]: <info> (eth1): exported as /org/freedesktop/NetworkManager/Devices/2
Jun  7 20:44:31 roman NetworkManager[4574]: <info> (eth1): now managed
Jun  7 20:44:31 roman NetworkManager[4574]: <info> (eth1): device state change: unmanaged -> unavailable (reason 'managed') [10 20 2]
Jun  7 20:44:31 roman NetworkManager[4574]: <info> (eth1): bringing up device.
Jun  7 20:44:31 roman NetworkManager[4574]: <info> (eth1): preparing device.
Jun  7 20:44:31 roman NetworkManager[4574]: <info> (eth1): deactivating device (reason: 2).
Jun  7 20:44:31 roman kernel: [ 3010.639973] eth1: rxqlen 0 --> 10
Jun  7 20:44:31 roman kernel: [ 3010.640099] eth1: rxqlen 10 --> 20
Jun  7 20:44:31 roman kernel: [ 3010.640167] eth1: rxqlen 20 --> 30
Jun  7 20:44:31 roman kernel: [ 3010.640235] eth1: rxqlen 30 --> 40
Jun  7 20:44:31 roman kernel: [ 3010.640255] eth1: rxqlen 40 --> 44
Jun  7 20:44:31 roman kernel: [ 3010.661375] eth1: ax88772 - Link status is: 0
Jun  7 20:44:31 roman NetworkManager[4574]: marshal_object: assertion `g_variant_is_object_path (path)' failed
Jun  7 20:44:31 roman NetworkManager[4574]: failed to marshal parameter 1 for signal DeviceAdded
Jun  7 20:44:31 roman NetworkManager[4574]: <info> (eth1): carrier now ON (device state 20)
Jun  7 20:44:31 roman NetworkManager[4574]: <info> (eth1): device state change: unavailable -> disconnected (reason 'carrier-changed') [20 30 40]
Jun  7 20:44:31 roman NetworkManager[4574]: <info> (eth1): carrier now OFF (device state 30)
Jun  7 20:44:31 roman NetworkManager[4574]: <info> (eth1): device state change: disconnected -> unavailable (reason 'carrier-changed') [30 20 40]
Jun  7 20:44:31 roman NetworkManager[4574]: <info> (eth1): deactivating device (reason: 40).
Jun  7 20:44:31 roman NetworkManager[4574]: <info> Policy set 'System eth0' (eth0) as default for IPv4 routing and DNS.
Jun  7 20:44:31 roman NetworkManager[4574]: <info> Policy set 'System eth0' (eth0) as default for IPv4 routing and DNS.
Jun  7 20:44:32 roman NetworkManager[4574]: <info> (eth1): carrier now ON (device state 20)
Jun  7 20:44:32 roman NetworkManager[4574]: <info> (eth1): device state change: unavailable -> disconnected (reason 'carrier-changed') [20 30 40]

Тоже касается и USB->Eth адаптера в N800:
Bus 005 Device 003: ID 0525:a4a2 Netchip Technology, Inc. Linux-USB Ethernet/RNDIS Gadget

Проверьте с libdbus-glib-0.94-alt2.

(В ответ на комментарий №6)
> Проверьте с libdbus-glib-0.94-alt2.
Теперь обнаруживает.