hi there !

i'm having a problem here, whenever I'm trying to change a css class, it's not applying.
for instance I'm trying to change the colour of my links for pda devices.
therefore I opened mj_xhtml.css in template/pda and did this small change.



but it's still the same colour that is showing.
mobile : samsung galaxy spica on android and nexus one on android too.

any idea ?

You need to change the following file to have changes reflect in iPhone:

root/templates/mobile_iphone/jqtouch-src/themes/apple/theme.min.css

If you can view the site in Firefox using either pda.yoursitename.com or a user agent switcher then firebug can be helpful for finding which css code is controlling the piece you need to change. Hopefully that will also be the css being used by the mobiles you mention.
help.joomla.org/files/joomla_firebug_tutorial_part1.swf is a very useful tutorial and although it is not the current version it gives techniques which allow you to see what css is doing and change it on the fly without changing your site. There is a second part to this tutorial at
help.joomla.org/files/joomla_firebug_tutorial_part2.swf

better late than never.
thank you all for your replies 
will tell you how if it works as soon as I try it