I had the same problem. I couldn't see my local drives when connecting my home laptop running Windows 7 to my work computer running Windows XP using Remote Desktop Connection.
I finally fixed it by doing the following:
1) Open the Remote Desktop Connection on my local machine, click on the Local Resources tab, select the More button at the bottom, UNCHECK ALL local devices and resources and then click the OK button.
2) Click the General tab and then click the Save button.
3) Click the Connect button, sign on to the remote computer and then disconnect.
4) Repeat step 1 again except this time CHECK the local resources you want (I just checked my local disk drives) and save the changes.
5) Connect to the remote computer again and your local drives should appear.
I'm guessing the problem had something to do with the RDC settings on the local machine getting corrupted. Clearing the local resources, saving the changes, connecting to the remote host, disconnecting, selecting the local resources again and then saving the changes seems to have fixed the problem (for me anyway).