Thank you for posting to the NI Discussion Forums.
While I have not used SDI-12 with LabVIEW, I would be happy to do whatever I can to help you out with this application. Did you have a specific question or a general question that I can help answer?
My understanding of SDI-12 is that it is a serial communication that takes place at 1200 baud over a standard RS-232 port. Our VISA API does allow for 1200 baud transfer rate so our serial devices along with LabVIEW should provide a good application setup. The fact that SDI-12 seems to use 12 V signals could make some type of voltage translator necessary. As long as there aren't any other quirks to the implementation of SDI-12 that I don't see, I think using LabVIEW and a serial port would suit your app just fine.