PalmDubai.net has a nice comparison article on the upcoming Treo 680 smartphone and the old Treo 650 smartphone. Although both phones looks the same except the antenna, the article explains further on the improvement.

As a long time Palm Treo 650 user, you might be looking into the new Treo 680 in order to stay with the Palm OS. The most notable differences on the 2 smartphones are the antenna, doubled memory (64MB) and the buttons design. The article can be summaries into:

• The Treo 680 is slightly smaller, thinner and without the antenna. the author feel that it is more comfortable to hold Treo 680 compare to Treo 650. (He is also a Treo 650 user)
• The QWERTY keyboard although looks the same, but it responds better.
• The smaller capacity of the Treo 680 battery (1200 mAh) doesn't seems to be affect the battery life, when compare to Treo 650 (1800 mAh)
• The sim slot is now hide under the battery instead of the top, bad for those who swap phone frequently but this should not be a problem to most of the Treo users.

You can further read more about the buttons functionality from the article here