My name is Christi.  I'm 29 and was diagnosed with an arachnoid cyst to the right of the midline in the posterior fossa.  I received this information originally 5 years ago at the onset of thyroid problems.  I had a headache for about 1 week - that completely knocked me on my butt so I went to the doc, he sent me for some blood work and a CAT scan and upon receiving the results, told me of the cyst.
Of course it was in passing that he mentioned it and only referred me to a neurologist when I requested it.  The neurologist said I shouldn't worry about it because it was not the cause of my headache.  In the mean time I was still hurting from this headache so I went to a chiropractor and let me tell you this - wrong thing to do.  The pain go worse after being cracked and was awful.
The bloodwork revealed hyperthyroidism/Graves Disease and I had surgery to remove my thyroid. 
Well, 5 years later - I had not "worried" about the thing in my head that wasn't supposed to be there.  I had another killer headache that lasted about 3 weeks (of which the last week was the worst and revealed contraction like pains from the lower right side of my head that radiated up over the top of my head and ended in the upper right area over my temple with a tingling sensation...almost like a firework starting from the ground...I finally went to the doctor.  He immediately treated me with migraine meds of which I didn't respond to (imitrex nasal and some other kind of quick disolve tab under my tongue).  I was in so much pain I was having a hard time articulating the feeling of pain so I stopped him, asked him to sit back down and proceeded to tell him my symptoms.  We ended the visit by sending me home with a sample / prescription of stadol (sp?) - some kind of nasal spary narcotic.  He told me to come back if it wasn't better over the weekend...I went back on the following Monday.
He finally sent me for an MRI and of course they noted the arachnoid cyst.  Since most everyone has had an MRI or will have one, it was an excruciating process for a headache sufferer - the sounds that machine made when it was taking the pictures was awful.  I thought I was going to split open like a banana.  When I finally got the results (a week and a half later) my doctor explained to me the size of the cyst and suggested it might be the cause of my headache (HA! - I just might be!)  My GP referred me to a neurologist (which happened to be the same one that already told me it was not a problem) and requested an appointment.  I have called his office 2 times and have yet to receive a response - he won't even see me because he saw me 5 years ago and already told me there was nothing wrong with me and that I've probably had this my whole life and never had symptoms - that it was an incidental finding - I would have never known it was there if I hadn't had the CT scan.  Funny thing is that I had the CT scan because of head pain - were they NOT supposed to find something that was causing the pain?  Didn't they find something that was "abnormal"?  Doesn't that mean that it's not really incidental?
I have seen countless doctors over the years for various other problems such as muscle pain, joint pain, sleeplessness, numbness, headaches, swelling of my hands, depression, etc.  Now I read other peoples stories and see that all of these symptoms are typicall of an AC sufferer.
I have had migraines since I was a child of 3 or 4 years of age.  I remember them that early.  I remember the pain and getting sick and having to lay down in bed with a cold rag over my face to help the it possible I've had this darn thing in my head my whole life.
In retrospect, it seems my cyst isn't as large as most other people have mentioned, however, I don't believe I'm asymptomatic or that "it's just in my head" the doctors have already told me.
Thank you for your site and allowing me to share my story.