This seems to me to be a nasty release blocker. As documented in the Tk bug that Victor cited (thanks!), the crash occurs in Cocoa Tk when an input method prefix "dead key" combining character is typed in a Tk text field. So, for example, with the US Extended input method on a US keyboard, to type ñ, you can type the two keystrokes: Option-n n.  Or for an ä: Option-u a. Typing the leading dead key causes Tk to crash with the "index out of bounds" exception, bringing down tkinter and IDLE with it (and users losing their edits).  There is a workaround for users to enter these characters without a crash.  OS X also supports a postfix "dead key" combining sequence; ñ -> n Shift-Option-n and ä -> a Shift-Option-u.  The postfix forms do not cause a crash with Cocoa Tk 8.5 and the expected character is stored in the field, although the screen may not get properly updated.  But, I suspect of the two, the prefix method is much more commonly used and, in any case, it is not reasonable to warn users that they risk a crash if they enter a common keyboard sequence.  (The other common method to enter characters not available on the keyboard is to use the system Character Viewer which gives access to all Unicode code points.  Unfortunately, Character Viewer input doesn't seem to work with Cocoa Tk 8.5.  That would seem to be a major deficiency in Cocoa Tk's support for full Unicode input.)

This problem is exhibited in the Apple-supplied Cocoa Tk 8.5 shipped with OS X 10.6 and the Apple-supplied IDLEs (/usr/bin/idle2.6 and /usr/bin/idle2.5) suffer from the same vulnerability.  So this is not a new problem in the ActiveState Tk 8.5.9.  The Tk bug report on the SourceForge tracker was opened and accepted nearly a year ago and, although subsequent comments and activity have raised the priority to 9 (critical), there is no indication that a fix is forthcoming soon.  I am pinging both at the SF tracker and on the Tcl Mac list and will propose some alternatives for 3.2 here shortly.