I had a closer look at the Telstra Thomson modem up the street today. It's a Thomson TG782T and the internal settings are accessed by typing http:// 10.0.0.138 into a browser window. This brings up a Windows 7 style interface window with lots of graphics and links to the configuring of various limited options.
There appears to be no option for varying the power, so we might assume that this is at 'full throttle' by default - nothing to change there. Other options allow changing of the default channel (11 in this case). This might be worth exploring because we can't immediately rule out the possibility that going outside brings the laptops into the influence of another wireless router nearby on the same channel. (Of course if you are in a rural or semi rural environment that can probably be dismissed altogether).
I was a bit perturbed and indeed puzzled to see that this one was set by default to work in Europe, but I could see no obvious way of changing it, so I stayed clear of that one. I'd still like to research the possible implications of this in the local situation 
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I was dismayed to see that the antenna does indeed appear to be fixed, so we can't experiment in that area as originally hoped.
FWIW I did check on my laptop last night using NetStumbler. The signal strength from the Thomson modem was, in my observation, usable from the adjoining house, and it was registering back in my office about 60 metres away and through another house between the two locations.
For the moment, it seems that trying another wireless channel might be the only option available in respect of 'settings' for the device, and given the apparent finality of the aerial fitment, it may well be worth experimenting with a corner reflector along the lines of that suggested earlier in the thread by seacue. This will direct the radiation pattern of the antenna outwards and away from the apex of the V but it is at the expense of signal strength and usability in all other directions. This needs to be weighed into the local requirements.
Trying a USB modem on the laptop could also be worth a try; I haven't any data or experience on the relative effectiveness of these vs the internal wireless modem / antenna combination. Others have reported an improvement, so this is also a possible worthwhile alternative. Like all such devices plugged into USB ports, they are vulnerable to accidental mechanical damage to themselves or the computer itself. Care is needed.
The radio amateur in me is uneasy, but I think I've pretty well exhausted the more obvious possibilities as I see them. It's back to the corner reflector and other options already suggested, but I would like to rule out co-channel competition and interference from any nearby wireless network first.
More than enough from me .......
Good luck
FOR
(in verbose mode 
- apologies are tendered)