It's never safe to assume that 'they' checked it out. Whoever they may be, that you don't specify, because you don't know. Some birthers are racists, some genuinely believe that he wasn't born in the US based on questions about his past.
If anything, I kind of like the fact that there are people out there who aren't afraid to suffer extreme public ridicule just to ask basic questions such as this.
But yes, quite a bit of racists were bound to latch onto it.
That said, I never once thought he was anything but a US citizen.
Also, it appears some are calling into question aspects of this new birth certificate.
Read all about it.