When sharing photographs from your iPhone using iPhone 3.0, you've probably been using the "Share" feature which automatically attaches the selected photos into the body of an email. Turns out that if you use a different method, you'll get the photos in much higher resolution.
iPhone 3.0 allows you to share photos in two ways:
1. Select photos, Press "Share", Automatically opens email, Send
This method automatically makes all the photos 800×600.
2. Select photos, Press "Copy", Manually open email, Paste photos in body, Send
This method makes the photos 1600×1200 (and 2048×1536 on the 3GS).
That being said, if you're looking for a higher resolution photo to share with people, use the copy feature instead of the share feature.