I've been looking at your website on and off for around 9 months.  I have 7 year old twins - one of which has an AC and the other has down syndrome (no AC).  How did we find out..  my son started to drag his right leg when running and his right foot's position was way to the right.  The orthopedist thought he might have a tethered spinal cord... well, we found out about the arachnoid cyst through the full spinal MRI.
As mentioned in the subject, his AC is located in the most popular place and it's about the size of a plum.
It explains so much that we've been through with him... yet, it's still hard to believe that all the below symptoms can be contributed to the cyst.  I know that you wanted a list of symptoms so here goes:

o  auditory processing delays
o  gross motor planning delays
o  balance issues
o  oral motor planning issues
o  low tone in abdomin and hands
o  tight heel cords (extreme)
o  tight hamstrings (extreme)
o  vision problems (been in glasses since age 2)
o  attention issues
The problems I have even telling you these symptoms is that my son was also a premature twin.  Born at 33 1/2 weeks and weighing 5 lbs 5 oz (yes big for being so early) so some of the symptoms listed could also be due to these problems.  Many of his symptoms didn't present until 3.5 years of age...
Nevertheless, I thought you should know since, as you have stated, there isn't very much informaton.  The good news is that his school is giving him all the help he needs and our pediatrician and neurosurgeon take everything very serious... much more serious than the neurologist that gave us the final diagnosis after our orthopedist was at a loss.  We are going to John Hopkins for his neurosurgeon.  He will be doing most of the care - an MRI every 9 months to monitor the size... no contact sports is the only constraint at this time.
Thought you might be interested.