The W800i has exactly the same headphone jack as the K750i - the two phones have identical hardware.  However, the headphones you get with the W800i have normal headphone jack connection so you can use any phones you like with them.  I just got a pair of W800i headphones for my K750i, way better than then the standard K750i ones and only £25.

IMO the K750i is easily the best all round phone on the market at the moment.  There are other phones with similar functionality (eg Nokia 6680 or N70) but they're nearly twice the size!

Michael.