Yesterday, I tried some testing with N1.

My purpose is to set my N1 as
NORMAL Phone to save some battery life or avoid others to 'exploring' your N1

. Possible? Yup.... simple...
To downgrade N1 superphone to normal classic phone
1) Get and install Task Killer from Market
2) Disable data connection via cellular
a) Dial *#*#4636#*#*
b) Phone Information -> Menu -> More -> Disable data connection
Now the N1 WILL NOT connect to Internet
or)
Make new bogus APN and set it as default.
or) Edit the current APN, and change the APN to the bogus APN, example celcom3g -> celcomg3

3) Open Task Killer and KILL launcher...
Now your N1 only can use Dialer/Phone only.. Yes, you cannot access any other menus or apps at all.
If you bored the classic phone mode, reset the N1 to get back to original state; Push Power + vol_down + trackball buttons simultaneously . Don forget to correct the apn setting if you want the data connections (GPRS, EDGE, UMTS, HSDPA) back.
BTW, do it at your own risk
Thank you.