I've actually used Google translate to infiltrate sites so-called protection that was designed to avoid prying eyes from seeing their cloaking activities. People think they're being clever and secure but nothing short of a full round-trip DNS check can be perfect.
See the original blog post where Matt Cutts explained how to do it:
Also, witness these recent threads about Google IPs:
Google? Is that you?
Google Test-Bot: Google-Test2
Bing, Ask, Yandex and all the rest you might allow need to be validated as tightly as possible too.