How can I evaluate or speculate on a part's production lifetime for long term part sourcing?
There are no guarantees, not even for the 68HC11. Ask your supplier what the policy on obsoleting parts is. Usually they send a notification with a last buy date. You'll have to buy sufficient parts to cover the remaining production years, or at least until you have a redesign ready. Depending on your relationship with your manufacturer (read: how many parts you purchase per year) you may get an early warning.  

On one occasion, for a custom IC the manufacturer didn't discontinue the part, but raised the price to such ridiculous levels that we decided ourselves to stop production. This was an IC produced with an older process which didn't have much production anymore.

Like Olin says, look out for second sources. If your supplier discontinues a part you may still find it at other manufacturers. But scrutinize datasheets. Sometimes second sources aren't exact copies, and the details may need a engineering changes in your design. If you're lucky this is just a resistor value, if you're out of luck this could be an extra resistor.
Also, changing manufacturers may also imply a different price, and a (much) higher price for the part may be a reason for a redesign, especially if you're running large production.

edit
Mike mentions the Flash memory market as notorious for its volatile availability. This is probably due to continuous advances in the field, especially in memory size.
Also expect short life times for emerging technologies, like OLED. I've had OLED modules becoming obsolete before we finished our design!



Further reading
NXP sample product discontinuation notice