Okay, but I've gotta ask...
Is the storyline you started for Halloween '08 gonna be done by Halloween '09? I thought some of my arcs ran kinda long, but damn! 
Well, during the "Halloween Caper 2008 Arc", it's actually several arcs... it all blends together. There are 4 arcs actually. While all 4 take place during the mall, and technically apart of the 2008 Halloween Cameo Caper, it's not one arc. Each arc is designed to run a particular idea.
Home Again. Home Again? was a segue arc to release some exposition about Nemi and her past, as well as getting everyone out of the graveyard to the Mall.
Find What You Need Here is the beginning of the Cameo Caper. Resolves the missing item that was taken from The Crossover Wars. Closes a plot point.
No Rest for the Soul is really the set-up for the Zombies and several other plot points, ending with Nemi's crystal getting jacked.
No Rest for the Dead, Either is a play on the previous arc name, which I do frequently, but also caries the bulk of the zombie stuff. It also brings up the Nemi and Bodil thing, where you are not 100% sure of what happened and who's who, etc. Ends with a bang.
The current arc
Altogether Apart, it's basically taking off from the last part, and is on it's way to wrap things up... meaning that when things are wrapped up (as far as the Cameo Caper is concerned), then a new arc will begin... but to give you a hint, they will STILL be in the mall when that happens, but will also move along in a different direction, as well as close some other plot points as well as set up new ones.
So, unlike Ardra, P.S.I.'s story runs consecutively. Rather than "Story. Story. Story. Story." P.S.I. is more like "Story: Story, Story, Story." Though I'll still have some non-consecutive points.... but I think you get what I mean.
Also, when I started PSI's inclusion into the 2008 Halloween Cameo Caper, I was VERY careful NOT to make it an official "halloween" arc. I wrote the entire script (or planned in my head, rather), to simultaneously run the PSI story, as well as the story for the caper.
Though I admit, I should work to shorten the bits, but during this 2nd major crossover event (unlike The Crossover Wars), I am actually releasing a whole lot more PSI Story intended plot points, peppered all throughout!
Also, as an extra hint, I'll be gearing up to introduce a new character before the end of the current arc!