Not necessarily true. On a slide phone, unlocking is generally easier because its just one action (slide up) instead of two. Some nokia phones (slides and bar phones), have a lock switch on the side which allows you lock and unlock with ease with just one click.
Generally, I don't mind the side to unlock. But when it comes to listening to music and using the iPod functionality, it gets in the way far too often. As far as I can tell, the only thing you can do with the phone locked is change the volume when listening to music. Every time I want to change a track, skip a song, change artist, I have to slide to unlock. That gets old fast. At least on Sony Ericsson phones, you can press and hold the volume up/down to skip tracks forward and backward. I don't see anything like that on the iPhone, unless i'm missing something?