Why is my new mid-2011 Air dropping wifi signal which old 2008 MacBook can hold? And is this related to my complete machine failures? I'm on my third mid-2011 MacBook Air in less than a month and this weird wifi problem may be related to the sudden, black screen bricklike nature of its predecessors.

The Air works fine when in the same large living room as the NexusLink 5631 router, but when I go into adjacent rooms, including, irritatingly, the one where my desk is, it repeatedly loses and regains its wifi signal. (Sites don't load in the browser, it stops being able to play music to the speakers in the living room, it stops being able to find the Time Machine backup drive in the living room, etc.)

My several year old MacBook does not have this problem and we haven't changed the router setup. This lost wifi signal problem is purely an issue with the Air.

The first two Airs I had suffered catastrophic failures. The pattern I can reconstruct is that at after getting them and using Time Machine to restore them to have all my old files, I had put them on my desk in the other room and put them to sleep in there overnight for the first time. In the morning they would not start, would not even display any error. The folks at the Genius Bar couldn't figure out what was wrong with them (as far as I know could get no response from them at all) and so exchanged them for new machines.

When Airs #1 and #2 failed they had been connected to 
1) a power supply that was connected to a power strip plugged into an outlet;
2) an external Nocent LCD monitor LM965WA with their lids up, displays unmirrored;
3) a powered Aquagate Monospace USB 2.0 7 port hub;
4) through the hub, a powered down HP Deskjet 920C printer; a powered up Brother HL2140 laser printer; and a Fitbit base station.

They had been connected to Time Machine on a Drobo in the other room, adding to the same history as previous machines.

Note that all of the above had been used without issue, apart from dropping wifi signal, prior to the overnight machine deaths. Also note that the old MacBook performed great for months and months in this setup.

With Air #3, we got a lot more cautious. 
We started a new sparsebundle to give it its own Time Machine history, on the principle that perhaps a history going back to 2008 was too much for it to handle over the air.
I have successfully put Air #3 to sleep overnight in the living room (where the router is) plugged into a new power cable.
I have also done so in the other room on my desk, also with new cable, using the same power outlet, but not going through the surge suppressor.

Air #3 has the same wifi dropping problem as the prior two Airs, so I'm wondering if this is an issue I'd have with any new machine or if I have received three defective machines.

My QA process to see if Air #3 will die overnight like it's pals is going to involve step by step letting it sleep overnight reintroducing one at a time the elements above, postponing the surge suppressor and Fitbit to last in the process.

However, it is unclear if the wifi issue is a cause of the failure (hard to see quite how when the machine is asleep and not trying to connect to, say, do an enormous Time Machine backup) or merely a coincidentally occurring problem.

Troubleshooting suggestions or pointers to others with either of these problems would be gratefully received!
Is your old MacBook (the one which doesn't drop connection) running Lion or a previous OSX release?

The reason why I'm asking this is that I suspect it might be an issue with Lion, and not with the Air.

I have an early 2009 17" MacBook Pro that never dropped Wifi while at home, ever. Since upgrading to Lion 1 week ago, I lost Wifi 3 or 4 times, but only for a short period (it automatically reconnects after a few seconds).

I am confident that it's not a router failure, as one of the time my MBP dropped wifi I was downloading a file on my wife's iMac (over Wifi) and it didn't drop.

Fwiw, my router is a 2nd-gen (dual-band) Time Capsule.