Well, the T68i looks a little different, same basic shape, but the styling is different. It's also badged Sony Ericsson.
The features are pretty much the same, but the T68i will support MMS (Multi Media Messaging). This is like SMS (text messaging) but a lot more free form. Send sound, pictures (real photos) and even video in the future. The text is colour and you can change the font. You can even change the timing on the display of the message, e.g. have a screen that says Happy Birthday, then auto replace it with the name of the person you sent it too, or perhaps a picture of yourself. This will be the next big thing. Imagine text messaging without the 160 character limit or a limit of text for that matter.
A camera will be available for the T68i- Com cam 2 (there's one for the T68 - Com Cam 1, but this 2 has more memory in it and allows you to use the phone's screen as a view finder - 1 doesn't). Clip this on the bottom of the phone, take a picture and MMS it to your mate, or to an e-mail address. Good ah?
If you already have a T68 and wished you got a T68i I the only real difference is software. All that needs to happen is for you to get a flash upgrade.