Hail storm several months ago suddenly accelerated plans to replace my roof and dying siding, and I am now nearly $13K richer from my insurance agency ready to start replacing my roof with a general contractor I really like and lucked into (PROTIP: Find a contractor whos old and doing this for fun, they ain't pushy). I told him that before I did, I wanted to try and see about getting mounts for solar panels installed while the roof was off, simply because I want to mess around with small solar systems and simply put, going cheap and used is one of the only ways these solar systems make economical sense.
My problem is- I have no idea what panel I'll go with, but I assumed from research that the mounting racks could make a lot of things fit provided it all could be bolted together. But a company I just spoke with told me that the racking needs to fit the panels exactly, that I cannot simply have the mounts ready for whatever material I scrounge up. I know some people here work in the industry; is there anything I can install now while the roof is off, so I can have a little cell later on?
All of the panel mounts I ever saw went on top of the shingles. I'd be surprised if you could get anything to seal if you have to put shingles around mounts. That would also mean that people would only be able to get panels installed every 30 years when they have the shingles off.
Not to mention, I wouldn't want to shingle a roof with all those obstacles.
^ This
And don't assume it has to be roof-mounted. If you have room in the yard, it can be better oriented and easier...everything, on the ground.
So.. your roof was destroyed by hail, and you want to put some far more fragile panels on top of it?
Might want to make sure they are insured in some way!! 
One thing I did figure out recently (not that it applies to what you are doing): I have always liked the idea of having batteries with a solar system (makes them FAR more practical), yet many installer shy away from them. I did a bit of research and found that typical battery installs are in the $15,000-$20,000 range, which is a bit harsh. In looking more into batteries, one of the comments was that the latest style of batteries are better than they used to be, they even have a 10+ year lifespan..... $15,000+ for 10 years!... that's $1500 a year (over $100/month), JUST to pay for the batteries. Even with obscene CA electricity prices, that is a very hard sell!
Now I know why so many seem to shy away from battery installs!
In reply to aircooled :
If grid tied then batteries don't make a lot of sense unless you have a high risk of losing grid power like hurricanes and such. In my case, my plan is enough batteries to last two nights worth of power and then dump money into overkill amount of panels so that even on cloudy days I'm making enough to charge them up. I also plan on putting extra money to make sure the house is very well sealed and insulated.
In my mom's case she loses grid power for hours often even without hurricanes. We've talked about doing a small battery setup that could run her AC, refrigerator, well and a few lights. She would cut power usage on everything else. She doesn't have a lot of sunny space but maybe a small panel set up for when it goes past a couple of days. I think both of us are influenced by Hugo when we lost power for a month. We would take showers at a friends house in town, go to bed when it got dark, and haul a bucket of water from the river to flush our toilets.
There are mounts that attach to a standing seam metal roof that don't penetrate the roof, and they don't create a dam that catches leaves/water.
If you are starting with a fresh roof, this might be an option.
In reply to GIRTHQUAKE :
You most definitely can install mounts for future solar, it's done all the time. It's a more difficult now that there is a lot more options on the physical sizes of modules, but you don't need to be exact, just in the ballpark. The mounts attach to the roof, and then the rails attach to the mounts. The modules sit on the rails, and there is some wiggle room on where the modules sit on the rails. You just want the two rails to go through the middle-ish part of the module, not on the ends. Most modules now range from about 69"-75" tall, and 40-41" wide. If you are trying to fit 3 plus rows on top of each other, it could be a bit of a gamble, you may be limited on what will fit later. But 1 row would be a non issue and 2 not much harder. I'd recommend mounts with L feet (which attached the mount to the rail) that are adjustable Vs. fixed like some mounts. That would give you a bit more flexibility. As for the mounts themselves, it depends on roof type. With tile it's much better to install with a new roof. With comp it's not as big a difference, but having one contractor responsible for the roof and mounts at the same time is a big plus. I'd pick a popular module that's not an oddball size and use that for your reference when setting the mounts.
Now, if you are planning on landscape orientation mounting, it's easy, since most modules are in the 40-41" wide like they have been for a long time. Spacing would be easy to set, just plan the rail length on the longer side in case you get longer modules. Just make sure the module can be short side mounted, but I think that's pretty standard now.
