Is there a way to change the default enter button on a Windows Phone 7 keyboard to Done or some other key like Send or something like an iPhone?
You can set InputScope in the text box to control what is displayed in SIP. Many of these include changes to 'enter'.
InputScope
You can see images of all different areas (yes, many of them are the same) at http://www.wpfied.com/2011/05/input-scopes-in-windows-phone-7/