The zoom slider on the Diamond2 and Touch Pro2 will allow you to zoom in Opera Mobile, Mobile Internet Explorer, and Photo Gallery. And as I discovered today, it also will make the text larger and smaller while you’re in Outlook Mobile (email or other messaging). It basically takes on the same function that you can find in Start>Settings>System>Screen>Text Size.

Update: As a Guest commenter points out, you can also use the Zoom slider in Word, Excel, and Adobe Reader.