Voice of experience talking…

On the new 3750X switches, you cannot stack a “LAN Base” unit with any other 3750 switch (including 3750, 3750G, 3750E or 3750X) other than another LAN Base 3750X. If you try (even with a 3750X running IP Base), the stack won’t come up.

This is different than other 3750 types. For example, you can stack “IP Base” 3750s with “IP Services” 3750s and it works fine. This scenario isn’t officially support by Cisco however.