FYI, this patch works great under .net 4 without modification. Given that Pageant is the ubiquitous SSH agent for windows, it's a shame that it can't be included. It's as simple as:
string host = "myhost";
string username = "joeschmo";
var agent = new PageantProtocol();
var conn = new AgentConnectionInfo(host,username,agent);
var client = new SshClient(conn));
(Insert appropriate exception handling of course...).
Robert