Zeep did indeed do what I wanted, and I found a site that hosts ASP.NET for free for 90 days, so I can now set my street cleaning days from my phone!
So if I have it set for the South side of the street (reminding me to move the car on Tuesdays and Fridays), my calendar looks like this:
And then I go somewhere and when I come back I park on the North side. So I text "cleander n" to 88147, and the reminders jump to Mondays and Thursday, (excluding any day in the green holiday calendar):
Zeep recognizes my phone number and uses the "cleander" string to route everything following it to my ASP.NET page in an HTTP POST request.
When the message gets to my site, the "n" or "s" populate the calendar with reminders on the appropriate days, an empty text clears the calendar completely (useful for going on vacation), and anything else will result in an error response.
I've saved "cleander n" and "cleander s" as message templates on my phone, so now any time I park it's just a few quick clicks.
Of course it will all fall apart in 90 days when my AspSpider account is deleted, but in the meantime maybe I'll be able to find some other free hosting option: all I need is to be able to run some sort of code in response to a POST request; the code is in C# right now, but it's short, and Google has APIs for Python and PHP and Java and whatnot as well, so I should be able to find something that works.
Update: Okay, Google's AppEngine will totally do what I need. I'll try it out tonight. It's fun and amusing to cobble together a baroque solution like this all to accomplish something that would take no effort at all if stupid T-Mobile would let me run a program on my phone in the first place. but also sort of depressing that I have to.
Update Deuce: This makes me feel a little better...apparently I'd have to do something similar on the Apple phone, as they don't provide a calendar API. Android does, but it's undocumented, though I would assume they make syncing with Google Calendars pretty easy so maybe it's less work. Palm gets it right.