What mounting structure can I use for corrugated steel roof?

Just to clarify: Your metal roof is flat? No pitch?

If so, then you should contact the roofing manufacturer and see if they make a flashing that matches the profile.

If you can't find the exact roof jack / flashing, you could try this pipe boot:

http://www.bestmaterials.com/masterflash_sizes_and_materials.aspx

You would pair that with a 1.5" x 4.5" stand-off - using an appropriate number of them to attach to the purlins and distribute the weight of the solar array.

 OK, that makes more sense. Are you in a marine environment? If not, you could run a piece of 1 5/8" galvinized unistrut vertically (parallel to ribs) and attach it to each purlin. You could put a continuous bead of Sikaflex 1-A sealant under the strut. You would not need a flashing and you would be able to attach your L-feet to the Unistrut slider nuts, thus having some vertical adjustment as well.

If you are in a marine (salty) environment, it's not advisable to have the aluminum L-foot in contact with the galvanized strut, since it will corrode quickly. If you are confident with your caulking skills and use high-quality caulk, you could attach a 3" stand-off directly to the purlin and seal it to the steel panel as you suggested, without a boot / flashing.

If you are in a wet area and can't risk a leak, then go with the flashing on top as well - pole barns work quite a bit as the seasons change and the various interactions of wood, steel and aluminum could eventually cause the unflashed joint between the the solar attachment and the roof to leak.

By the way, hanger bolts can also be used in the flat section of your panel - but a 2x4 is probably not tall (1.5") enough to get a good attachment - here's one more idea: Drill through the steel and the purlin, then just use all-thread as a "hanger bolt" with fender washers top and bottom to clamp and seal the joint. The hanger bolt would then hold the L-foot at whatever height you wanted.

You could also check the 3" Pro-Solar stand-off

S-5 Corrubracket for corrugated roof are available at Greentech Renewables.

These are nice, refined standoffs!  I especially like the butyl rubber insert.  However, they would not have worked in my case.  Some of my rafters lined up with the major ribs, so I had to mount feet to the purlins which are only 1 3/4 thick.  A single lag screw would not be able to carry my uplift forces.

I ended up using Unirac 2 piece standoffs (no affiliation) with butyl tape underneath and silicone caulk over the screw heads.  This allowed me to have 2 screws holding each foot, and (since I used all Unirac hardware) allowed me to use the manufacturers load calculator, PE approval letter, etc.  (I am a PE, but not experienced in this area.)  I finished the installation late August.

So far no leaks or modules falling off the roof, and that's after huricane Irene!

I have a corrugated Metal Roof with no Sheathing / Deck . I do have Metal Structural Rafters. I am using S5! Protea Brackets . What should be the minimum Metal thickness for Installing Solar. ? I see some Engineers saying 20 Roof gauge metal and some saying at least 22 guage ?