Jeff,
Pine boards are fine. Shavings can be bad because so much surface area is exposed to the air. Not true with boards. Besides, you'll need to seal the pine with something anyways and pine resin is not known for being able to migrate through most finishes (unlike cedar).
Really, most solid wood could be used for snake cages. I would avoid redwood and cedar, or any board material that is can be used on outdoor furniture or decks without protective treatment.
Pine is pretty cheap and hard to beat.
Also consider those edge-glued pine panels. I have two scrap pieces that have been outside for over a year now. The boards have checked and warped badly from the 120* temperature difference they've seen in the last year but none of the edges have shown any sign of coming apart. In a reptile cage that is a more stable environment you should not have any problem. Do let the material acclimate to your indoor conditions before making the cages, however.
And really seal the heck out of it.
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Current snakes:
0.1 Gonyosoma oxycephala - (Silver/Yellow)
1.2 Gonyosoma oxycephala - (Green)
7.6 Gonyosoma janseni - (Black)
0.1 Gonyosoma janseni - (Black & Tan)