With -wrap none, a sufficiently long line crashes the session.
Running OpenSuse 11.4
Crash occurs with tcl/tk 8.5.9 and 8.6b2
Crash occurs with several different window managers

Following program exhibits the crash:

package require Tk
set log_file

proc main {} {
   text .t -width 80 -height 30 -bg white -font {arial -12} -wrap none
   pack .t -fill both -expand 1

   for {set i 0} {$i < 10000} {incr i} {
      update idletasks
      .t insert end "A"
      set fd [open $::log_file w]
      puts $fd $i
      close $fd
   }
}
main

Note that the log file records how many characters were added
before the crash. Experimenting with different font sizes shows
that the crash occurs when the line exceeds about 8050 pixels.

This is slightly similar to bug 624732 - but that is reported as fixed...

Is this still happening (preferably on a recent system)?

Can you provide a stack trace at the crash point?