Very surprised it wasn't added in the first update. Such a glaring and obvious need for many cell-phone users (well, to the people I am around at least )
You can do it through email...it's not ideal and kind of a hassle but it's the only to do it till 1.1.3 comes out and on top of that, it's a free service.
Basically all you have to do is send your friends emails. If you don't know their sms email address (number@vtext.com for verizon for example) you can send them a text at number@teleflip.com and teleflip will get it to whatever network they are on. If they reply back, it'll be sent to your email or if they're smart they'll reply directly to your number.
Since the iPhone doesn't allow you to send multiple SMSs (...yet) I made a special text message email for everyone of my contacts. It'll also come in handy if I'm near to going over my limit for texts.