Here's the word from Planet Half-Life about recent server listing issues:
The recent problems with displaying the server list within Half-Life has been identified as a load related problem with the Half-Life Master Server. Valve has provided a temporary fix to the master server to prevent this problem from occurring. This temporary fix limits the maximum number of servers that will be returned to the Half-Life client. Currently this limit is set to approximately 2000 servers. If your favorite server is no longer showing up within the Half-Life client, please add it directly to your favorites list to gain access.Of course, you can use GameSpy, which will list more servers (from the gspy master server).
Valve will be addressing this problem in upcoming update for Half-Life and the update will eliminate the need for the limit of returned servers.