Okay, now I'm on a working network, but:

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$ time darcs pull -a --verbose --debug-verbose http://allmydata.org/ 
source/tahoe/trunk
Beginning identifying repository .
Done identifying repository .
Beginning identifying repository http://allmydata.org/source/tahoe/trunk
URL.copyUrl (http://allmydata.org/source/tahoe/trunk/_darcs/format
           -> /Users/wonwinmcbrootles/playground/allmydata/tahoe/ 
trunk-hf/darcsYDYGo6)
URL.copyUrl succeeded
URL.waitNextUrl failed: http://allmydata.org/source/tahoe/trunk/ 
_darcs/format /Users/wonwinmcbrootles/playground/allmydata/tahoe/ 
trunk-hf/darcsYDYGo6 HTTP response code said error
URL.waitUrl http://allmydata.org/source/tahoe/trunk/_darcs/format  
succeeded
URL.copyUrl (http://allmydata.org/source/tahoe/trunk/_darcs/inventory
           -> /Users/wonwinmcbrootles/playground/allmydata/tahoe/ 
trunk-hf/darcsKImKXd)
URL.copyUrl succeeded
URL.waitNextUrl succeeded: http://allmydata.org/source/tahoe/trunk/ 
_darcs/inventory /Users/wonwinmcbrootles/playground/allmydata/tahoe/ 
trunk-hf/darcsKImKXd
Done identifying repository http://allmydata.org/source/tahoe/trunk

darcs failed:  Not a repository: http://allmydata.org/source/tahoe/ 
trunk (Failed to download URL http://allmydata.org/source/tahoe/trunk/ 
_darcs/inventory: )
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and yet, I can download that file:

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wget http://allmydata.org/source/tahoe/trunk/_darcs/inventory
--16:26:41--  http://allmydata.org/source/tahoe/trunk/_darcs/inventory
            => `inventory.1'
Resolving allmydata.org... done.
Connecting to allmydata.org[207.7.153.190]:80... connected.
HTTP request sent, awaiting response... 200 OK
Length: 70,176 [text/plain]

     0K .......... .......... .......... .......... .......... 72%    
82.64 KB/s
    50K .......... ........                                   100%    
59.59 KB/s

16:26:42 (74.82 KB/s) - `inventory.1' saved [70176/70176]
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dupe