Just spoke with a roofer. He said that polyiso was a pain to put on a roof because you need to put plywood over it. Looking around a bit, I see that some vendors have a product which is polyiso bonded to plywood or OSB, called nailbase or nailable polyiso. See http://c.ymcdn.com/sites/www.polyiso.org/resource/resmgr/technical_bulletins/tb106_jun30.pdf I don't see it for sale anywhere, though. Has anyone used it? Is it cheaper than just separate plywood and polyiso? Where does one get it?

Nailbase is an alternative which we have used with good results. Works best if you do not want a vented area above the polyiso. It can be difficult sealing between the panels so be prepared to caulk or foam the joints, and use a good first quality tape like Siga or 3M 8067. Since you cannot seal it as easily as you can polyiso which is easy to tape, it's worth installing a good air barrier on the old roof deck - ether a peel & stick or a good taped fabric.

You can order it from a commercial roofing supply company in the thickness you want, or from a SIPS panel company.

Atlas makes insulation roof boards with venting: http://www.atlasroofing.com/engineered-ventilation

Thanks for the replies. I did find a directory that lists some local sources for several brands of nailable polyiso within 20 miles:

http://www.buildsite.com/product/nailable-base-3
http://www.buildsite.com/product/energyguard-composite-board-insulation-gaf2
http://www.buildsite.com/product/acfoam-nail-base-insulation
http://www.buildsite.com/product/nailboard
etc. etc.

In my case, this is above a well-vented attic, so air sealing the roof deck isn't important.
I plan to put R30 on the attic floor, so all the polyiso would do is make the AC system's life easier; our ducts run through the attic.
On hot days, our living area ceiling hits 95 degrees, and the outside shingles hit 177 degrees.
(Dunno what the attic temperature is, I need to measure that.)
Then again, we're going to use Energy Star shingles, maybe the ~50% increase in reflectivity vs. our current shingles will cool the attic enough.

This has been hashed over elsewhere, see http://www.greenbuildingtalk.com/Forums/tabid/53/aff/14/aft/82842/afv/topic/afpgj/1/Default.aspx

It's not clear to me why they can't nail shingles right through 1/2" polyiso foam.

Jonr, I think they can but each fastener might be sunk to a different level. If so, then the finished roof might not look smooth. I say this because the foam will not resist the fastener as much as OSB would.

Also, if you're mounting solar panels, maybe polyiso alone wouldn't support the rack as well? I don't know, I should ask.