You may want to investigate IF This Then That and maybe their recipe that allows you to automatically forward a e-mail message to a phone via SMS. This recipe may not fully work for you, depending on your needs, but this is a start.
ifttt is truly a godsend when it comes to connecting different services.
The Email-SMS gateway that Google Voice uses formats email addresses like this in the messages that it sends to you:
12125551212.12015550189.{some-random-string}@txt.voice.google.com
The first phone number there is your own, the second is the other person in the conversation. The random string is just that, a bunch of letters, numbers, and punctuation with no apparent rhyme or reason.
However, this email address is re-usable. I just took a SMS message email that I'd received a couple of days ago and grabbed the address. Then I sent a new email message to that address and the recipient received it as a SMS message from my Google Voice number.
(You can't leave the random string out; it's probably some sort of hash to prevent random SMS sending.)
So, it's no help for the first time you send someone an SMS message, but if they send/reply to you (and you're using the Email-SMS gateway feature of Google Voice) you can save the address used to their contact record and can simply use email to send them SMS in the future.
Note: this only appears to work for the Gmail account associated with the Google Voice account sending the message. (Thank you Larry.)
Best Answer
Only the cellular phone carrier can decide whether you can send SMS via email or not. Here's how to reach people on the most popular carriers:
If you have some programming skills, you could make use of an SMS API (such as ZeepMobile or Twilio) and build a database associating emails to an SMS phone number. Then you could send emails to "phonenumber@yourappname.com". The app would receive the email and redirect the message to the appropriate phone number.