Thanks to everyone for the explanations.  The note about two-way 
communication is very important and something I haven't been thinking 
about enough -- it isn't just that the computer can "hear" the wireless 
router, but it also has to have the power to be heard by it effectively.

This reminds me that I wanted to ask what you know about the wireless 
capabilities of various laptops and netbooks.  It seems to me that my 
recently-purchased HP Pavilion g7-2022us does nowhere near as well as my 
Asus Eee 1005HA at maintaining a connection to a wireless router.  If the 
signal seems weak, the the HP is having trouble, I can use the Asus.  Is 
that because of reception or transmission?  Has this been measured for 
various machines with results posted to the web?  It is a very important 
aspect of performance, but I have never read specs on this and I can't 
find any now.  I'm disappointed that the HP performs so poorly compared to 
the much cheaper and older Asus.

I remember that Negroponte's XO (OLPC) laptop was supposed to do perform 
well with a weak signal (despite its low price).  I saw him say on a TV 
interview that this good performance was "because of the ears" (also known 
as wifi antennas).  Here are the ears:

http://news.bbc.co.uk/nol/shared/bsp/hi/image_maps/07/1179000000/1179824483/img/navigation2_416.gif

This led me to ask myself an obvious question:  Where are my ears?  None 
of my laptop/netbooks have external ears.  How good are their internal 
ears and how do they compare with those of the OLPC XO-1+ machines?  I 
want numbers!  ;-)

Mike