Which would be correct:
Bill is making a cake for Karin and I
or
Bill is making a cake for Karin and myself
Neither. "Bill is making a cake for Karin and me." One trick you can use to quickly figure this out is to remove "...Karin and..." so that the sentence reads, "Bill is making a cake for me." In your sentences, this would work out to "Bill is making a cake for I" and "Bill is making a cake for myself." The latter is using a reflexive pronoun to refer to the subject, which is Bill. This doesn't work because you are not Bill.
Update:
Here are a few links that may be helpful:
http://grammar.quickanddirtytips.com/between-you-and-me.aspx http://grammar.quickanddirtytips.com/myself-grammar.aspx