Can you use old water heater tanks for rain harvesting?

There are a lot of used water heaters at my dump (for free!). I'd like to know if they are usable as rain storage tanks? Thanks for the advice.

I'm also interested in this. I have one old hot water heater and two brand spanking new ones (someone I knows solution to everything is to throw money at it) one of them is propane and has a striped part the other is natural gas and has a clogged port (or something) from having been ran on propane for a few weeks. They say right on them not to return them to the store but to the manufacturer so I guess it was just more convenient to buy a third brand new water heater. They've been covered in tarps and sitting and I don't have a bill of sale. I do have an interest in thermal siphoning and water catchment though

Perhaps someone out there on the mighty internet can answer this question for the OP and I?

I can't think of a reason why they couldn't be used. Might be easy to do plumbing from them too.

I can't think of a reason why not either. I think most people just don't have access to a bunch of them (in Phoenix they get recycled for scrap metal so they are hard to find). It could be too that the ratio of heaviness to amount of water stored keeps some folks from using them (as opposed to some kind of plastic tank/barrel say). But hell, if you have 'em, use 'em. And like Miles said - they'd probably be easy to plumb.

And report back here with pictures! You guys might start a new permie trend.