403 Forbidden is one of the HTTP Status Codes. As per the definition:
The server understood the request, but
is refusing to fulfill it.
Authorization will not help and the
request SHOULD NOT be repeated. If the
request method was not HEAD and the
server wishes to make public why the
request has not been fulfilled, it
SHOULD describe the reason for the
refusal in the entity. If the server
does not wish to make this information
available to the client, the status
code 404 (Not Found) can be used
instead.
I imagine that your repository is something like http://server/repo/project/ so try browsing that URL with your regular browser. Maybe is not related to Subversion authentication but HTTP authentication.
I know it is not a good answer, but it might give you a hint to expand the question :)