Using cast iron on a glass range I currently rent, and the only problem I have with the house we're in is that it has a glass-top range.  I absolutely love my cast iron cookware, and use it as often as possible.

I've heard a few reasons not to use cast iron on glass, including:


You can scratch the glass if you're
not careful
It's very easy to break the glass if
you drop it
You can melt the glass top
(mentioned in linked thread)
It can be hard to minimize hot spots


I'm not very concerned about (1) and (2), as I've already been using cast iron on glass for years with no problems (I'm very careful when I do use it).  

While researching (4) I came across (3), something I had never heard before.  So, I now have a few questions:


Is melting the glass really
plausible?  This seems unlikely to
me, as a quick search revealed that
lowest melting point for common
glass types is 1500C or higher, a temperature I have no plans to reach - but
I'm not a physicist or a chemist, nor do I know what kind of glass is used to make a glass top stove.
Is a heat diffuser effective on a
glass range?  I have experienced a
few hot spots on my large (12")
skillet, and obviously when
experimenting with my two-burner
griddle.  Would a heat diffuser help, especially with the two-burner (which I've all but given up with on
glass for now because it's so
uneven).
Any suggestions for safely and
effectively using cast iron on a
glass-top range (short of being very, very careful when moving the pans)?
I also have a glass-top stove at home. So far, I really like it: the heat is very even and there is good heat transfer, provided the pan makes good contact with the surface (one skillet I have has a bit of a lip, which makes it heat more slowly).

I've slapped skillets and pots full of water around on mine, and have yet to break it. I'm probably more careful than I would be on a metal range, but it seems sturdy.

But to your points:

1) There is no way cast iron could melt the glass. Iron melts lower than does glass (1200*C vs. 1500*C), so before you manage to melt your stove, your pan will be a puddle. It may, however, be possible to deform the stove top if you let an empty pan heat for some time. I don't see why this would be more of an issue with cast iron than with any other piece of cookware.

2) A heat defuser will work fine. Don't get the kind that is designed for use with gas, but anything else should work fine.

3) The burners on my stove are smaller than my skillet, so I find I need to move the skillet around to heat the edges. Also, flat-bottomed pans and pots seem to be much more effective on the flat glass top, as conduction seems more effective than radiation here. I polish the bottoms of my pots to get better heat transfer, although this is just me being anal.

Also: cast iron can't scratch glass. Glass is much harder than iron (see this wikipedia article), glass having a 6-7 on the Mohs scale, iron having a 4.