da http://www.quirksmode.org/blog/archi..._mobile_1.html

Handheld stylesheets

@media handheld, which is supposed to allow you to define special style rules for mobile devices, is supported only in mobile mode; and not in desktop mode. As far as I’m concerned Opera has made a mistake here: the device is a mobile phone, therefore @media handheld should be supported in both modes.
In the current situation Opera denies web developers the opportunity of optimising their sites for mobile phones, and it also denies there’s any difference between wide, spacious computer screens and narrow, cramped phone displays. I feel it’s wrong on both counts.
More in general, the handheld media type is useless right now. Apart from the Opera problems WebKit and Blackberry don’t support it, and only one out of three tested NetFronts does.
IE supports it, but it supports @media screen simultaneously. Although IE has been much reviled for this support, I think that in practice it’s not so bad. You can still use handheld stylesheets as an overrule for normal stylesheets, and that may be enough to help your mobile users.