CAN bus ESD protection
The point of ESD protection is to give high voltages an easier path to ground than through the sensitive components on the board. In general, the protection devices will thus be connected to the circuit's ground.

If the case is solidly grounded (either to an earth ground or to another common ground, such as a vehicle chassis), a case may be made for including it in the ESD protection scheme. The idea is to dissipate as much of the power of the discharge as possible in a small amount of time, and the added metal and common ground will help that. However, when introducing an enclosure into a circuit, additional factors come into play:


Ground loops - if you're not prepared for this possibility, the enclosure should not be considered as a ground
Connection capacity - your connection to the chassis should allow for high (albeit momentary) currents
Interference - if the design ends up connecting the ground plane to the enclosure, there is a new point of entry for undesirable signals that may disrupt the operation of the circuit


The end result is that it's much easier to avoid involving the enclosure in any ESD protection schemes. There may be advantages to doing so, but there should be careful consideration involved.

Here is an EDN article on ESD mitigation measures for high-performance serial busses: http://edn.com/design/analog/4314519/Maximizing-EOS-and-ESD-immunity-in-high-performance-serial-buses.