Last week, really spent some good time troubleshooting why LPT1 could not be mapped to a network printer queue. In this case, a user could use "net use lpt1 \\server1\printershare1" on this local machine, but not on the terminal server. Login script was fine, everything went fine, except for mapping the LPT1-port.
Turned out that the user had Administrative rights on the local machine, and was just a "Domain user" on the terminal server. (Whether this is good or bad is out of scope for this post... :-))
Did some googling, and first came up with this one: "Unable to Map (Net Use) LPT1 in Windows XP". After reading some more, it turned out that M$ itself has a knowledge base article on this: "Non-administrators cannot remap an LPT port to a network printer".
Different from what Microsoft says, I have found this issue only to occur for LPT1; other local ports can be mapped to network shares without a problem...