Pondrew I'm sorry I Disagree....
Just for info, I have 2 C500's here and if you keep the screen on there is virtually no difference in the battery discharge time between display on and display off.
As an example, my C500 ran in a test the other day (Re the poor battery topic in this forum) with the display on (not the backlight) from midnight and at 7pm the following day (19 hours) and the battery was still on a full 4 bars while the C550 had turned off due to battery drain.
I have had my C500 since day 1 and I have always had the display timeout option set to 'never' and my phone lasts for several days (and I also have GPRS permanently connected with email polling every 30 mins, 24 hours a day). This option on the C550 (without the GPRS connected and email polling) runs the battery completely down in 10 hours.
One hell of a BIG difference.
Phone History: 7110 > T68 > T68i > 7650 > SPV > E100 > 6600 > P900 > Treo 600 > E200 > C500 > Jam > SDA > C550 > QTEK8100 > iMate SP5 & JasJar > HTC MTeoR > Samsung i600 > HTC S620 > Dash