Honestly, I don't know what kind of datacenters you guys are using, but if theft is a concern, you've got other problems.  Personally I have had items go missing at a data center, however they were cheap tools and I'm sure taken without malicious intent.  Even at extremely low end data centers, like Verio in SF (converted dog kennels), equipment removal (from premises) and cage access (if you have a cage) is supervised.  At SAS70 DCs this would be impossible.  Further even the most expensive bitcoin mining equipment doesn't cast a shadow on the price of 50% of the servers you'll see in racks.  My company, for example, has two racks of 40 1U's, each 1U costs more (just the CPUs in fact are more) than the 400gh/s unit and would be easier to sell on ebay or the like if stolen.

I dunno where the cancer and paranoia of the bitcoin community comes from, but alas its everywhere all the time and appears to be self-refining.