v3.4 problem on iPad 3rd gen – custom admin icons for CPT

I use a simple custom plugin to adds a custom post type to use with a slideshow plugin. In addition to setting up the CPT it also adds a custom 32x32px icon to the top of the CPT page and an image sprite for menu icons (B/W version & colour version for hover & selected states) in the admin using a function to add the following css to admin:

#menu-posts-slide .wp-menu-image {
    background: url(<?php echo plugins_url() ?>/cc-slider-custom-post-type/images/slide-icon16.png) no-repeat 6px -16px !important;
}
#menu-posts-slide:hover .wp-menu-image,
#menu-posts-slide.wp-has-current-submenu .wp-menu-image {
    background-position:6px 8px !important;
}
#icon-edit.icon32-posts-slide {
    background: url(<?php echo plugins_url() ?>/cc-slider-custom-post-type/images/slide-icon32.png) no-repeat;
}

It worked fine in WP v3.32 on all browsers I tested on, and after updating WP to v3.4 still appears fine on desktop browsers & iPhone 3GS, however the icons no longer show correctly on the iPad (3rd Gen).

Is the menu output differently for the iPad? I’ve not managed to do a “view source” to find out.

Is there a better/recommended way to add the icons for a custom post type to get the hover/selected state?