As it says in the following forum post:
In the context of a TextBox, readonly
allows the user to set focus to and
select and copy the text but not
modify it. A disabled TextBox does not
allow any interaction whatsoever.Use ReadOnly when you have data that
you want the user to see and copy, but
not modify. Use a disabled textbox,
when the data you are displaying is
not applicable in for the current
state of a dialog or window.
Taken from: MSDN Forums