Or it could be something slightly different.
If you have SQL named instances then the SMP will first query the server on port 1434 via UDP. Once the server responded the connection will be established on port 1433.
Now, I had this issue with a customer a few weeks back, and it turned out that the SQL Browser service just needed a little restart to get back to work. With that done the server was back in workign order, connecting to SQL with no fuss.
BTW, on that case the worse thing was that it happened whilst the customer upgraded to MP1, so it first felt like an installation problem when it was just a SQL connectivity issue.