Sending is easy (from desktop email to phone text message). I think there was a discussion somewhere on here about it - for example, to send to someone on Sprint, send the email to
phonenumber@messaging.sprintpcs.com (remember the 160 character limit on text messages).
Receiving text messages on a desktop unfortunately isn't as easy... I don't think it's possible with free services.
For 2-way conversations with a desktop, it's probably easiest to use a chat program - i.e. Palringo has windows and mobile versions of their chat client. Thus send an email/text message to the mobile phone, tell them to meet you on Palringo, and you'll be chatting in a few minutes.
Hope that helps, --TonyK.