Confirmed here too.
I have an apt-proxy on 192.168.2.2:9999
Apt-updates work fine from the local machine, but fail when attempting to connect from a local machine. It looks like Robert's comment is correct - apt-proxy is failing to parse the url correctly. 
Compare the following log segments:
Failing log (connecting from a remote machine).
Notice that a dynamic backend is created for ttp: (nonsense)
2006/06/03 12:16 BST [Channel,0,192.168.2.11] [debug] Headers: Host: 192.168.2.2:9999, Cache-Control: max-age=0, User-Agent: Debian APT-HTTP/1.3
2006/06/03 12:16 BST [Channel,0,192.168.2.11] [debug] Request: GET http://192.168.2.2:9999/ubuntu/dists/dapper/Release.gpg
2006/06/03 12:16 BST [Channel,0,192.168.2.11] [simplify_path] simplified path from http://192.168.2.2:9999/ubuntu/dists/dapper/Release.gpg to http:/192.168.2.2:9999/ubuntu/dists/dapper/Release.gpg
2006/06/03 12:16 BST [Channel,0,192.168.2.11] [debug] Request: GET http:/192.168.2.2:9999/ubuntu/dists/dapper/Release.gpg backend=ttp: local_file=/var/cache/apt-proxyhttp:/192.168.2.2:9999/ubuntu/dists/dapper/Release.gpg
2006/06/03 12:16 BST [Channel,0,192.168.2.11] [debug] Adding ttp: backend dynamicaly
2006/06/03 12:16 BST [Channel,0,192.168.2.11] [debug] Created new BackendServer: http://ttp:
2006/06/03 12:16 BST [Channel,0,192.168.2.11] [debug] backend: ttp: [<apt_proxy.apt_proxy.BackendServer instance at 0xb765c60c>]
2006/06/03 12:16 BST [Channel,0,192.168.2.11] [Fetcher.activate] (ttp:) servers:1http:/192.168.2.2:9999/ubuntu/dists/dapper/Release.gpg
2006/06/03 12:16 BST [Channel,0,192.168.2.11] [file_ok] check_cached: /var/cache/apt-proxyhttp:/192.168.2.2:9999/ubuntu/dists/dapper/Release.gpg
2006/06/03 12:16 BST [Channel,0,192.168.2.11] [debug] cached: 0
2006/06/03 12:16 BST [Channel,0,192.168.2.11] [fetch_real] Consulting server about /var/cache/apt-proxyhttp:/192.168.2.2:9999/ubuntu/dists/dapper/Release.gpg
2006/06/03 12:16 BST [Channel,0,192.168.2.11] [Fetcher.activate] (ttp:) servers:1http:/192.168.2.2:9999/ubuntu/dists/dapper/Release.gpg
2006/06/03 12:16 BST [Channel,0,192.168.2.11] Unhandled error in Deferred:
2006/06/03 12:16 BST [Channel,0,192.168.2.11] Traceback (most recent call last):
          File "/usr/lib/python2.4/site-packages/twisted/web/http.py", line 557, in requestReceived
            self.process()
          File "/usr/lib/python2.4/site-packages/apt_proxy/apt_proxy.py", line 1400, in process
            self.fetch()
          File "/usr/lib/python2.4/site-packages/apt_proxy/apt_proxy.py", line 1482, in fetch
            (dummyFetcher, 0, running,), None)
          File "/usr/lib/python2.4/site-packages/twisted/internet/defer.py", line 182, in addCallbacks
            self._runCallbacks()
        --- <exception caught here> ---
          File "/usr/lib/python2.4/site-packages/twisted/internet/defer.py", line 307, in _runCallbacks
            self.result = callback(self.result, *args, **kw)
          File "/usr/lib/python2.4/site-packages/apt_proxy/apt_proxy.py", line 1451, in fetch_real
            fetcher = dummyFetcher.apEndTransfer(fetcher_class)
          File "/usr/lib/python2.4/site-packages/apt_proxy/apt_proxy.py", line 463, in apEndTransfer
            fetcher = fetcher_class(req)
          File "/usr/lib/python2.4/site-packages/apt_proxy/apt_proxy.py", line 317, in __init__
            self.activate(request)
          File "/usr/lib/python2.4/site-packages/apt_proxy/apt_proxy.py", line 616, in activate
            ClientFactory(self), request.backend.config.timeout)
          File "/usr/lib/python2.4/site-packages/twisted/internet/posixbase.py", line 392, in connectTCP
            c = tcp.Connector(host, port, factory, timeout, bindAddress, self)
          File "/usr/lib/python2.4/site-packages/twisted/internet/tcp.py", line 840, in __init__
            raise error.ServiceNameUnknownError(string="%s (%r)" % (e, port))
        twisted.internet.error.ServiceNameUnknownError: Service name given as port is unknown: service/proto not found ('').


And a local connection to the same apt-proxy instance which works correctly:
2006/06/03 12:13 BST [Channel,6,192.168.2.2] [debug] Headers: Host: 192.168.2.2:9999, Connection: keep-alive, If-Modified-Since: Sat, 03 Jun 2006 08:21:11 GMT, User-Agent: Debian APT-HTTP/1.3
2006/06/03 12:13 BST [Channel,6,192.168.2.2] [debug] Request: GET /ubuntu/dists/dapper-updates/Release
2006/06/03 12:13 BST [Channel,6,192.168.2.2] [debug] Request: GET /ubuntu/dists/dapper-updates/Release backend=ubuntu local_file=/var/cache/apt-proxy/ubuntu/dists/dapper-updates/Release
2006/06/03 12:13 BST [Channel,6,192.168.2.2] [debug] backend: ubuntu [<apt_proxy.apt_proxy.BackendServer instance at 0xb767104c>]
2006/06/03 12:13 BST [Channel,6,192.168.2.2] [Fetcher.activate] (ubuntu) servers:1/ubuntu/dists/dapper-updates/Release
2006/06/03 12:13 BST [Channel,6,192.168.2.2] [file_ok] check_cached: /var/cache/apt-proxy/ubuntu/dists/dapper-updates/Release
2006/06/03 12:13 BST [Channel,6,192.168.2.2] [debug] Verification skipped for /var/cache/apt-proxy/ubuntu/dists/dapper-updates/Release
2006/06/03 12:13 BST [Channel,6,192.168.2.2] [file_ok] Modification time:Sat Jun  3 09:18:11 2006
2006/06/03 12:13 BST [Channel,6,192.168.2.2] [file_ok] last_access from db: Sat Jun  3 09:41:22 2006
2006/06/03 12:13 BST [Channel,6,192.168.2.2] [file_ok] file is too old: /var/cache/apt-proxy/ubuntu/dists/dapper-updates/Release
2006/06/03 12:13 BST [Channel,6,192.168.2.2] [debug] cached: 0
2006/06/03 12:13 BST [Channel,6,192.168.2.2] [fetch_real] Consulting server about /var/cache/apt-proxy/ubuntu/dists/dapper-updates/Release
2006/06/03 12:13 BST [Channel,6,192.168.2.2] [Fetcher.activate] (ubuntu) servers:1/ubuntu/dists/dapper-updates/Release