Yes, it's possible if all the t-shirt images and all the design images are the same size.
The following instructions are for Photoshop CS4:
Click Window >> Actions.
At the bottom of the Actions window click the Create New Action button. Name it "Merge Designs" or whatever you like.
Click Record and merge one design into one t-shirt. This will presumably involve File >> Open on the t-shirt image, then however you would normally proceed (open, resize, position, etc.).
On the Actions window, click the Stop button (the square).
Click File >> Automate >> Batch.
In the Play section, from whatever Set you stored your action in, select the action. For Source, choose your folder of T-shirt designs. For Destination, pick a new (empty) folder.
I haven't actually tried this, you might have to fiddle with the actions. In particular, I would suggest that having the t-shirt image in the clipboard before starting and simply pasting it will greatly simplify your job (since you will presumably only run this batch once).
Hope that's helpful.