I am interested in how and when can carriers (and government) spy on us - when they can listen or track us when we use maemo/meego devices.

I have this "tablet mode" in my n900. Does this mean that when my n900 is in tablet mode - they cannot listen for my talks or track my location?

Not that I think I am so important so that there is a huge market for my talks - but I always use tablet mode when there is a wifi connection and I am kinda paranoid. When there is no wifi connection - I have a data only card - don't use cellular calls at all - only encrypted xmpp messages.

What about n9? Does it have some plugin which turns it to tablet mode? And does this tablet mode indeed work so that carrier indeed cannot get the gsm chip to work? I can imagine that gsm chip goes to sleep, but do we know how can it be awakened?
Can it be awakened by a signal from carrier?

First of all, what the operators and the government can do depends on the laws as well as the resources they have. Before I move on, may I ask? Why do you even have a phone? And why do you think wifi is more secure?

I use the phone as a mobile computer - I need internet connection to check emails and use instant messaging.
I have offline cached maps so I don't need internet for this.

I have checked my emails with tcpdump - mostly there is an encrypted connection.
Also I have checked that my IM connection is encrypted.
I am able to change my wireless NIC hw address.
But I am not able to change the IMEI, right?

what about laws - I am more interested in technical side.
also, I am not sure that those who spy really care about laws.

You can change IMEI, however its not recommended becouse its illegal in many countries so we don't go into this here. Most users are not more secure by trying a stunt like that.

What kind of encryption are you using?

Unless you use end-to-end encryption with sufficient cryptographic strength, the government can intercept your communication if it wants to.

Relating to your question, it means it doesn't matter whether you use GSM or WiFi. You need to encrypt your communication so that only the recipient knows how to decrypt it.

If you only want to hide your identity and do not care about your messages being intercepted, then something like using Tor network is recommended. But even that is not foolproof as shown by various research projects.

This is just my little understanding of the subject, the specific answer what measures you need to take depends on if you want to make your message unknown or your identity unknown - or both.

Just to make it clear, I don't think the "tablet mode" has anything to do with deterring the prying eyes of the government.

encryption - tls - to connect to mail server
ssl - with xmpp server.
But this is the same whether I use laptop or phone.
The kind of security I am concerned more is phone device specific.
I feel pretty safe about my IM and emails, but I know that carrier can track my location, and may be even listen talks by using phone's microphone.
And this is done by using gsm chip, that's why I am concerned about gsm part.

Just take off the battery, im sure they wont hear anything

by reflashing gsm firmware?

seriously I am not sure - there were devices with backup batteries and taking out the batteries was not enough to be safe. (: