Here's a forum thread where Nexus One users are reporting a similar issue:
My main problem is with Apple's dismissive response. Although at least they have responded at all, unlike Google.
The yellow blotches are apparently where the bonding agent hasn't fully dried. Probably because they're rushing them out the door a little too quickly. I've read that after a day or so of use they disappear.
Kinda funny though - Steve Jobs demoed iPhone 4 in his left hand, and many of the people in the advert are holding it in their left hand. Oops