In the long list of useful functions that WordPress provides for your development pleasure, remove_accents is a handy function that converts any accented characters to their ASCII equivalents. Here’s an example:

$swedish_string = 'Hej och välkommen till vår hemsida!';
$without_accents = remove_accents( $swedish_string );
echo $without_accents;

This echoes: ‘Hej och valkommen till var hemsida!’

This is a really useful function for sanitizing urls and when working with the filesystem in PHP.

There’s another example of how to use the function on the codex page.