I've found Twilio and Tropo so far as services to aid SMS and Voice apps.
Are there others?
I am interested mostly in real-time speech recognition (i.e. the user can speak things and the system can respond based on that).
Also, if anyone knows what the speech recognition quality is like, it'd help!
For reliable SMS delivery we tested Tropo and Twilio and Twilio won hands down... Better API imo (including callbacks that make things like knowing how long a call lasted trivially easy), they manage the sending rate so you don't have to (Tropo discards SMS messages if you send too fast).
On the other hand, Tropo has speech recognition that works very well (although not as well on real-world cell calls in less than silent environments) and has integrated Twitter support.
Both offer excellent support. Tropo is free for development, but Twilio is so inexpensive that's probably a non issue.
My more detailed writeup is at http://pardner.com/2011/04/tropo-not-ready-for-prime-time-went-with-twilio/
(Since the blog was written I've learned Tropo fixed Twitter support, so that's actually a very cool feature in favor of Tropo. Turns out you can mix n match, our app uses BOTH.)