I think it's very unlikely we'll see data plan rates go down. They may be advertised as lower but it will be with use caps etc.
If anything, (and we're starting to see this already) the carriers will migrate their voice minutes component prices into the data plan rates (and possibly refer to the combined package as something new). There was a story in the wsj not long ago talking about the challenge the carriers are having with college age (+/- few years both sides) who no longer use their voice minutes. They either use texting or socal networking apps like Facebook as their communication transport mechanism.
The only way we'll see total monthly subscription rates decrease is if there is some new compelling technology that arises that they can begin charging for. These companies are in business to make money for themselves and shareholders- and lowering prices does not easily support this objective.
I know some will say that lowering prices will drive a share shift, but in reality, this it much more difficult to do than it sounds - and ofter takes much longer than investors are willing to wait and requires a heavy investment in marketing to ensure awareness (current expense).
Also, given how expensive these phones are and the rate of change, I can't practically see the phone subsidized pricing model going away very soon. It's much easier for most people to pay an extra $10-15 a month than it is to come up with $300-$400 every 18mths.