Not logging this in the bugs section, but poking at the new webapp feature...
laplace:annex jtang$ git annex webapp -d [2012-07-26 19:02:50 IST] read: git ["--git-dir=/Users/jtang/annex/.git","--work-tree=/Users/jtang/annex","show-ref","git-annex"] [2012-07-26 19:02:50 IST] read: git ["--git-dir=/Users/jtang/annex/.git","--work-tree=/Users/jtang/annex","show-ref","--hash","refs/heads/git-annex"] [2012-07-26 19:02:50 IST] read: git ["--git-dir=/Users/jtang/annex/.git","--work-tree=/Users/jtang/annex","log","refs/heads/git-annex..731005d121426a38b206c4544da02cdb3b974974","--oneline","-n1"] [2012-07-26 19:02:50 IST] read: git ["--git-dir=/Users/jtang/annex/.git","--work-tree=/Users/jtang/annex","log","refs/heads/git-annex..d36d8d88847decc2320f0be22892ad94a8abe594","--oneline","-n1"] [2012-07-26 19:02:50 IST] read: git ["--git-dir=/Users/jtang/annex/.git","--work-tree=/Users/jtang/annex","log","refs/heads/git-annex..57bcddc14d03b61028f7002e2dabcc5181d74f3d","--oneline","-n1"] [2012-07-26 19:02:50 IST] read: git ["--git-dir=/Users/jtang/annex/.git","--work-tree=/Users/jtang/annex","log","refs/heads/git-annex..372aceaf49b60ebe31cc3fe2e52ba4fbe99c134f","--oneline","-n1"] [2012-07-26 19:02:50 IST] chat: git ["--git-dir=/Users/jtang/annex/.git","--work-tree=/Users/jtang/annex","cat-file","--batch"] [2012-07-26 19:02:50 IST] read: git ["config","--null","--list"] [2012-07-26 19:02:50 IST] call: open ["file:///Users/jtang/annex/.git/annex/webapp.html"] The file /Users/jtang/annex/.git/annex/webapp.html does not exist. git-annex: failed to start web browser on url file:///Users/jtang/annex/.git/annex/webapp.html
I would have expected the open command to open up http://localhost:port/ instead of a file on my machine. Anyway, its just an observation on the current state of the webapp feature, not expecting it to work enough for me to test it 
This could happen if you already had an old version of git-annex assistant running in that repository.
Try:
git annex assistant --stop ; git annex webapp
The last few commits fixed the webapp startup quirk, it doesn't work yet for me as the watch command is probably failing
it doesn't segfault or anything, it just states that the assistant command failed to start :P
I've been improving this code path, so make sure you're current. I just made it wait for longer than the 10 seconds it was waiting before giving up.
If it still fails, try running
git-annex assistant --debug --foregroundin one terminal, wait for it to start up, and then rungit annex webappin another. If that still fails, check if.git/annex/webapp.htmlexists.