The behavior occurs because ephemeral ports, temporary ports that are used for client server communications, are not closed upon use. Ephemeral ports are limited to a range of ports and are only valid for the duration of the connection.
In this case, on certain Microsoft Windows operating systems, SDK connections leave the ports in a LAST_ACK state on the vCenter Server and therefore the range of ports is exhausted after a period of time. When this happens, SDK connectivity fails until a port is freed.
This is not a VMware issue. This issue is caused by a race condition with the Microsoft TCP/IP driver.
Note: The preceding link was correct as of March 25, 2010. If you find the link is broken, provide feedback and a VMware employee will update the link.