Just because you don't have an Android phone or BlackBerry doesn't mean you can't text from your cellphone using your Google Voice number. If you've gotten an invite to Google Voice and you're an iPhone or non-smartphone user, you might be disappointed about not having clear access to the texting feature. However, you CAN use Google Voice SMS capabilities on any phone. Even if you do have an Android phone or BlackBerry, you can use this method to use the native SMS app on it. It's an inconvenient kluge, but it works.
First, log into Google Voice and configure it to forward text messages to your cell phone. When someone sends a text message to your Google Voice number, you'll receive the text on your phone--but not from the recipient's phone number. Instead, it will be a 406 number you've never seen before, with the person's name preceding the message (as pictured here). Add that number to your recipient's address book entry as "Other" or a custom label (like "GV SMS"). Each one of your text recipients will have a different 406 number.
From there on in, if you SMS that 406 number, your recipient will receive text messages from you--and it will look like they're coming from your Google Voice number. Their replies to any messages you send to that number will go back to your Google Voice number and come to you via the 406--meaning, your recipient never sees the 406 number. Like I said, it's a kluge, but it works.