> I was just thinking, why don't they
> make OTP packages out of some
> UV transmissive plastic?

Fair question; it would seem that transparent plastic would be cheaper and
easier than those expensive ceramic/quartz packages the manufacturers use.
There are, however, a number of problems that would have to be solved to
make "transparent" devices practical.

[1] Many types of transparent plastics are either opaque to hard-UV light,
    or are degraded by it (they either become opaque or lose structural
    stability).

[2] An IC's package must be able to withstand very high thermal gradients
    at the leads while remaining gas-tight.

[3] For small quantity runs, it's cheaper to produce chips in ceramic than
    to tweak all the necessary parameters for encasement in plastic.  This
    is why prototype chips are almost invariably produced in ceramic pack-
    ages (windowed or not).

[4] There isn't a terribly high demand for EPROM-based micros in transpar-
    ent packages.  Unlike chips such as optical sensors and such which need
    to be in transparent packages to be useful, the total demand for many
    types of windowed EPROM processors is in the low thousands; in products
    that don't require UV-erasability, opacity would be a good thing even
    when it wasn't strictly necessary.

If someone were to produce a micro with a built-in 1000 element optical
sensor (bar-code scanners, anyone?) it may be practical to undergo the
expense of a semi-UV-transparent package for all the micros (since a trans-
parent enclosure would be needed to use the optical sensor).  Most likely in
such a case, though, the device would only be good for a few program/erase
cycles before the case was too badly damaged (if you were careful to avoid
excessive exposure you might get a dozen or two).  Absent a compelling rea-
son to make the mass-produced micros transparent, though, I doubt strongly
that any manufacturer is going to bother.