I recently got an X41 and it's a really nice laptop. The screen is good (despite one stuck blue subpixel at the top center), though I haven't used it outside. Very light and thin, with long battery life (4-5 hrs, not sure exactly -- I haven't run it down yet) using the 8 cell battery (it does sticks out a bit at the end and add 0.5lbs relative to the 4 cell). Easy to use without the ultrabase, unless you need cd/dvd access -- I would recommend getting an ultrabase so you can load stuff from cd/dvd's, but you don't need to carry it around.
The X41 does warm up a bit, but I've no problems using it on my lap. The fan runs a far amount of the time but it's very quiet -- noise is not an issue at all IMO.
It's built really well, and the keyboard is great, especially for such a small notebook. (I like the larger keyboard with a pointing stick and no touchpad, and no windows key; even then the backspace key is a little smaller than I'd like.) I really recommend the X41 if you're looking for an ultraportable.