Search the PPRuNe archives, there are MANY threads about this, usually in PAX & SLF.
Bascially, what the CAA + FAA advise is for nothing - if the carrier that you are on says No. The fact that many new mobile (cell) phones have a 'flight' mode where the transmitter is off, so that you can use the games and PDA functions etc - most carriers will not accept that it IS turned off and will not want to see any device in use.
It's their aircraft and their insurance if it goes wrong! So keep listening for announcements and ask the cabin staff. If in doubt - don't!