There's a change in the pipeline that will be hitting the public Windows Phone 7 images at some point soon (post the current Beta), which removes the ScrollViewer from a TextBox's template.
What does this mean?
Basically, long TextBoxes will no longer scroll when you gesture over them - the gesture is ignored and there is no visual reaction. Tapping remains the same (the keyboard pops up) as does tapping and holding (the enlarged caret is shown, and you can use this to, sort of, scroll).
How do I work around this?
Most scenarios do not require scrollable TextBoxes, but if your's does you can either place the TextBox inside a ScrollViewer (though this can cause some texture issues), or preferably re-add the ScrollViewer into your TextBoxes template, so that it still scrolls as usual.