PhotoStream to a specified folder?

Not exactly. Here's what I do. I allow Photostream to auto-create events for each month. Once a month, I come along and launch iPhoto. I then drag the photos out of last month's event to a folder on my firewire drive. At that point, I sometimes delete the photos from iPhoto (to save space) then re-import them back into iPhoto (with copy to iPhoto Library switched off). I also disable uploading to Photostream from my Mac.

If you're looking for a strictly folder-based service, I might recommend eyefi. You pick up an eyefi SD card for around $50 and it comes with a free account. Don't bother paying for any upgrades. The free account transfers photos directly from your camera to your Mac whenever your camera is in range of wifi it knows about. Meanwhile, if you already have an eyefi account, you can download the eyefi app from the iOS app store. It can upload photos directly to a folder from your iPhone. You can set eyefi to autocreate folders based on date, etc. Eyefi is not nearly as automatic nor as seamless as Photostream.

There are other methods to get photos off your iPhone ranging from free to paid. I happen to like Photostream, even if I don't like letting iPhoto have the only copies of my pictures. The eyefi app is still on my iPhone, but I haven't launched it since Photostream became available.