I have to say I think your friend is being a bit passive-aggressive and using that to try to manipulate you. I mean, in the example you gave, that guilt trip as you said and making you feel bad -- she succeeded in getting you to do just what she wanted, and since her strategy worked she'll probably use it again and again.
You can try talking to her as Jan suggested (communication is always a good thing!), but I also think that if you stop paying attention to her little pouting routine, just pretend that everything is fine, keep smiling and talking to her as if nothing had happened, then after awhile she should get the message that you're not going to be manipulated anymore!
It could be that a sincere discussion and the realization that you're not going to play this little game anymore will help your friend wake up and maybe grow up a little bit. If you do your best to make it clear to her how you feel and she keeps behaving this way, I would really start to wonder how good a friend she is and if you really need to be around her.
I hope you and your friend can work things out -- but please don't let her continue to manipulate you!
** My poor neglected blog is here
RIP Gabby (9/12/2009) -- I miss you SO much. We'll never forget what a sweet girl you were!