I find the thing that defines a "Desktop replacement" to me, is neither size, nor features, but battery life. A lot of lappies tout an Athlon 64 or boggo standardo Pentium 1V which provide plenty of power, but can chew batteries up like nothing on Earth. The usefulness of such a device is the point on the graph where portability and the inconvenience of lugging batteries seemingly made of lead, cross - and that point shocks many, many people.
I use an Acer Aspire as a Desktop replacement (and a handheld Dell does the outdoor bit due to the above!) and I think it to be a good machine. However, you are never going to get the performance of a modern, compact and neat desktop, which still cleans up on performance over a laptop. It would be cheaper and easier to maintain and upgrade as well. If you really are stuck on a lappie, then go for it, but I would suggest that you have a look at what is available on the desktop front first. Also, have a look at handheld computers like I did. (I am wowed by mine. Flexible and powerful pice of kit with much functionality)
Conan