As an INFP, I certainly often feel boring, but then if I scrunch up my face with all the effort I can muster and try to be objective, I remember that I do have a good number of hobbies that have interest value, can have good conversations (both serious and bantering) and have more than once had a friend just stop mid-conversation and express something that can be vaguely paraphrased as telling me I am odd/interesting/unique.
However, I think UDog has hit upon the problem. INFPs are unassuming. Unassuming is often boring. Also, some of us tend to have issues interacting in anything other than a one on one setting, so... unless conditions are practically perfect, any potential for being interesting is locked away. Internally, the INFP in a group setting is too busy gasping like a fish to be interesting. Or... at least me. I believe some of the more extroverted types can function better with people, and draw attention through antics and general charisma, and as INFPs, we are keenly aware of the fact that we can't capture interest in such a way- whether or not we'd actually want to. (Afterall, it is contrary to our character to be in the spotlight...)
I think a more interesting question is how would an INFP overcome this while still staying true to his/herself? Have any INFPs found ways to successfully mitigate this? Do any other types have possible solutions?