Yes there's a known problem - the X3 series is not compatible with Snow Leopard.
There's an ongoing status report on the situation in one of hte Announcements at the top of the main X3 Discussions Forum page. Here's the current copy:
Attention Line 6 USB audio interface/device users: Mac OSX 10.6.x (Snow Leopard) is currently not supported by Line 6. We expect to be implementing support for Snow Leopard soon.
While you are free to try installing our software on this new operating system, we will not be guaranteeing issue-free operation on 10.6.x systems. Using Leopard OSX 10.5.x would be the recommended platform by Line 6 for Mac Users' productivity at this point in time.
For further information, please use the following link:
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OSX Snow Leopard User Group: