Sometimes I do not need Wifi (battery saving) but I do need Bluetooth for the GPS or UMTS network connection.
When I select offline mode *both* are off.
How can I set BT on and WIFI off ?

You can turn on bluetooth in flightmode with vabgeo's switchonbt:
http://switchonbt.garage.maemo.org/

The statusbar icon doesn't change to reflect that it is connected to something however.

I think the Wifi powersaving mechanisms of the N8x0 are very efficient. No need to turn Wifi off in order to extend the battery life.

I'm mostly using Wifi at home. So I in the first months after I got my N800 I used to put the tablet into offline mode while not being at home.

I stopped doing this already quite a while a ago and honestly I didn't see any difference in battery life.

(Though I should mention, that I have the Wifi Autoscan / Autoconnect features of my N800 disabled. I guess if you enable these additonal features this might increase power consumption.)

How do I set autoscan and autoconnect off ?
Is autoconnect possible per network ?

Er, you simply don't connect to any wi-fi access point. There's nothing more to it. This is what happens whenever I click on connection and select my BT phone instead of any AP.

In the connections settings -> "Search interval". I think the factory setting for this is every 5 minutes or every 10 minutes. I did set it to "never".

be sure also that modest isn't trying to get emails,
i got problems with this app some time ago... so i turned it off too

Thanks a lot, I now let Wfi on but with auto search to 'never' and Bluetooth to on but not visible, so I can use Bluetooth always when I use GPS applications or connect to mobile internet.

hmm, a quick test here shows that if something provokes the "select connection" dialog to pop up (say a rogue mail checker when a known wifi network is not within reach) will continually check for new networks while open.

in earlier versions, one could have wifi fully off (flight/offline mode) and still have bluetooth on via switchonbt. but diablo seems to have a script or something running that checks if bluetooth is on with offline mode active, and turns it off.