While the current iPhone data plan included 200 text messages in the price, this is not the case for the next generation iPhone. If you want texting with your 3G iPhone plan, you'll need to pay $5/month extra. While some of you may be willing to just pay the extra fee, there are few other options you may want to consider.
One good service is TxtDrop, which lets you access a webapp optimized for the iPhone and send free text messages. Just go to http://www.txtdrop.com/iphone. It is completely free because it is ad-supported by a small Adsense unit at the bottom. Simply input your email address (which serves as the "sender"), the number of your recipient, and the message. There may be some truncation for the text message because the TxtDrop URL is added to the end of the message, so just be sparing on your character use.
Aside from TxtDrop and similar services, each of the carriers offers the same kind of free texting. If you know the carrier your recipient is using, you can use one of the following, as listed by iPhoneFreak.com: "For Verison sending an email to [their 10 digit number]@vtext.com will
convert your message into a text message and go to that number.
AT&T uses [their 10 digit number]@txt.att.net and Alltel uses
[their 10 digit number]@message.alltel.com."
Of course you can't reply directly to these texts because they use a service number, not the number of the sender, but they do link to the sender's email address, so there is at least some option to continue the conversation. Another hope might lie in instant messenging services. As the applications for IM get better, hopefully the integration with SMS will also improve.
[Via iPhoneFreak]