The official line from the Rambus website is "Yes, however the system will run at the speed of the slowest module". However, there are always exceptions to the rule.
Memory compatibility is also affected by the design of the system motherboard. For example, Intel only recommends 600mhz and 800mhz RIMMs in there 840 motherboards.

The metal cover is a heat spreader made of aluminium. The heat spreader protects the memory and helps diffuse the heat generated by the faster access and transfer speeds.