Either svn.openwrt.org is down, or you cannot reach it.
Have you tried doing svn co svn://svn.openwrt.org/openwrt/branches/backfire/package/uhttpd/src uhttpd-src
from the command line?
Update: It works for me at the moment:
$ svn info svn://svn.openwrt.org/openwrt/branches/backfire/package/uhttpd/src
Path: src
URL: svn://svn.openwrt.org/openwrt/branches/backfire/package/uhttpd/src
Relative URL: ^/branches/backfire/package/uhttpd/src
Repository Root: svn://svn.openwrt.org/openwrt
Repository UUID: 3c298f89-4303-0410-b956-a3cf2f4a3e73
Revision: 38417
Node Kind: directory
Last Changed Author: jow
Last Changed Rev: 30807
Last Changed Date: 2012-03-04 16:54:55 +0100 (Sun, 04 Mar 2012)
So either it was a temporary glitch, and works for you now as well, or you cannot access the server from your site.