How to ask for rec letter for another program after verbal commitment to stay with my current advisor for PhD? I'm a senior undergrad, and I verbally committed to sticking around with my current research prof for a PhD at the beginning of the school year. However, I'm now really enticed by the graduate program of another school, and I'd like to at least apply to the other school.

However, I don't know how to ask my research prof--whose lab is the technically-juciest part of my CV--for a rec letter. He has already recommended me to 2 fellowships, so I know he has a letter ready for me.

Should I just not ask my advisor for a letter? How would a grad committee feel about the lack of input from the closest research adviser? I have a 3.8 and 3 years of hands-on experience in different labs (no publications) and student groups, so I feel like I have a decent shot at getting into the other school without my adviser's recommendation, but I'd like to get your thoughts.

More broadly speaking, is it okay for me to even apply to the other school? On one hand, I feel guilty about possibly reneging on my statement to work with him, especially since I've had some unproductive spats in his lab and I feel indebted to him for keeping me on. On the other hand, I don't feel guilty because he told me he doesn't have any long-term funding left for me (hence the fellowship applications). Moreover, one of the fellowship proposal's depended on the lack of progress in a certain field which my prof and I later discovered has been secretly progressed very rapidly by a government lab.
Absolutely you should apply to other programs, and your advisor should be encouraging you to do so.  If for no other reason, your advisor could be hit by a bus tomorrow, and then you'd be stuck without any back-up plans. 

Your advisor should have no qualms about you applying to additional programs. He should be willing to submit his LOR for you. Moreover, if you secure some interviews, it is such a unique opportunity to meet some other PIs in your field as well as students who are in your field. 

In sum, your professor should be totally open to doing this, just explain that you don't want to place all your eggs in one basket.  Good luck!