A useful tool to use to check all required ports are open is momNetChk.exe – This is available from the Microsoft Operations Manager 2005 Toolkit. You can download directly from Microsoft from http://download.microsoft.com/download/d/4/b/d4bfc32c-d8d2-4541-8356-4c6359eecbb4/MOM2005ResKit.msi
If you prefer to have a read before downloading visit http://technet.microsoft.com/en-us/systemcenter/om/bb498260. The tool is available below the Tools and Utilities section.
After you have installed the tool kit, a folder will appear under c:\Program Files named Microsoft Operations Manager Resource Kit. Do a search for momnetchk.exe and then double click the .exe, type in a machine name and click run scan. All tests should show green, if not save the results to a log file and check which ports were blocked.
A really useful tool.
I would recommend you run the tool from both, Server and Client and ensure the required ports are open. For example, if you open port 1270 TCP but not UDP you may experience issues such as the last contact column within MOM console being blank. To install on the client just copy the whole folder across to C:\program files on the client machine.