Some other turret models have been posted and people have been asking for the model for this one on the flickr set ( http://www.flickr.com/photos/7820200@N05/sets/72157626476156514/with/5614051491/ ), so I thought I'd post my progress. There's a lot left to be done, but the white shell parts are pretty much finalized. It's a complex model, and rendering will take a while, this is normal. You can change the $fn variable at the beginning to make smoother curves with smaller facets, but the larger the variable, the longer it will take to render. I've been working on other projects, but if/when I finish this, I'll update this space.
This model is based on careful measurement of images from screenshots from Portal, Portal 2, and Portal 2 promotional materials. It is actual size, as measured by taking screenshots in which Chell can be seen standing next to the turret, and assuming her to be average height for an adult female. This resulted in an estimate that the standing turret is 1.5m tall including the antenna, and the spheroid portion is 1m tall when folded.
Update: thanks to eric1000 for the pics of a 1/10 scale shapeways print!
Update 2: The design now has parameters for size (height of the complete spheroid, "full size" is 1000mm) and thickness of the shell in mm. This should make it easier to produce printable STL files. Also, the different parts are now modules that can be easily disabled at the beginning of the file.
Update 3: I've successfully printed all of the shell parts on my Makerbot with supports at 1/5 scale. Pictures have been added, and I've posted the STLs I used.
Update 4: I've added the black feet and updated the legs with small recesses to insert them into. The scad file is also updated, and includes the code for generating the printable plates.
Print one of each STL raftless with supports. If your supports wont stick, increase their flow rate. The body and arms are split into upper and lower halves, and you can use acetone to solvent weld them together.