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

-------
$ 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: )
-------

and yet, I can download that file:

-------
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]
-------

As a counter point, I have a darcs2 compiled tonight (2/15), and the inventory
is found in my case, and darcs starts downloading all the patches:

$ 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
wget -q --header=Pragma: no-cache --header=Cache-Control: no-cache -O
/home/mark/Haskell/tmp/darcs8Q6IRh
http://allmydata.org/source/tahoe/trunk/_darcs/format ; # Null Null AsIs
wget -q --header=Pragma: no-cache --header=Cache-Control: no-cache -O
/home/mark/Haskell/tmp/darcs0tZR9r
http://allmydata.org/source/tahoe/trunk/_darcs/inventory ; # Null Null AsIs
Done identifying repository http://allmydata.org/source/tahoe/trunk
wget -q --header=Pragma: no-cache --header=Cache-Control: no-cache -O
/home/mark/Haskell/tmp/darcsOS32T2
http://allmydata.org/source/tahoe/trunk/_darcs/prefs/sources ; # Null Null AsIs
Beginning reading inventory of repository http://allmydata.org/source/tahoe/trunk
wget -q --header=Pragma: no-cache --header=Cache-Control: no-cache -O
/home/mark/Haskell/tmp/darcsrlNXMD
http://allmydata.org/source/tahoe/trunk/_darcs/inventory ; # Null Null AsIs
wget -q --header=Pragma: no-cache --header=Cache-Control: no-cache -O
/home/mark/Haskell/tmp/darcs7klJcf
http://allmydata.org/source/tahoe/trunk/_darcs/inventories/20080108184749-92b7f-5df11c2d78599efdd56baece7488fa6a749d65c7.gz
; # Null Null AsIs
wget -q --header=Pragma: no-cache --header=Cache-Control: no-cache -O
/home/mark/Haskell/tmp/darcsRsox0Q
http://allmydata.org/source/tahoe/trunk/_darcs/inventories/20071016041556-92b7f-95c1d3dcb8a417ed1d884cbcb5b81a38173c15ed.gz
; # Null Null AsIs
wget -q --header=Pragma: no-cache --header=Cache-Control: no-cache -O
/home/mark/Haskell/tmp/darcseRfx9s
http://allmydata.org/source/tahoe/trunk/_darcs/inventories/20070823210315-92b7f-888fbf1b1867ccda8218ad9f955b835cbd474164.gz
; # Null Null AsIs
Reading inventory of repository http://allmydata.org/source/tahoe/trunk
inventories/20071016041556-92b7f-95c1d3dcb8a417ed1d884cbcb5b81a38173c15ed.gz

Zooko,

Which network library is your Darcs compiled with? Perhaps it's specific to
that, and I could re-compile for a counter-test.

How can I tell which network library is compiled into my darcs executable? 
Looks like curl:

string `type -p darcs` | grep -i curl

curl_multi_init() failed
curl_easy_init() failed
curl_slist_append() failed
curl_multi_perform() - no running handles
curl_multi_info_read() failed
curl_multi_info_read() returned unknown message
darcs/2.0.0pre3 libcurl/7.13.1
[Cleanup curl pipelining configure check.
[Fix configure check for curl_multi_timeout.
[[issue633] config detection of curl_multi_timeout presence for older curl
libraries.
[Fix curl version check for pipelining.
[Merge Curl and Libwww to URL module.
[Rework libcurl module: use multi interface, support pipelining.

Okay, I upgraded the the latest version of darcs-2 and the problem is fixed! 
For the record, here is the diff between darcs --exact-version of the failing
version and of the succeeding (latest) version:

Oh, weird -- the new version of darcs reports its exact-version as:

darcs compiled on Feb 17 2008, at 09:58:11
unknown

FYI, I believe this was a duplicate of a bug reported by Jason (not sure of
issue number).  It was just some stupidity on my part in error handling in the
URL module.  I failed on success, not on error...

David