I've knocked heads with mailto: in the past. Here's a few points that come to mind.
1) Each browser has a different implementation. You need to test.
2) The mail client on a machine varies. To be thorough you need to test whatever your audience has, or the most common mail clients (assuming you don't know what they have!). (That means using VM's, getting the tools licensed or having separate machines.)
3) Those two points above give you a big matrix of tests to run (IE, FF, Opera, Safari X all the emailers). If you automate your tests it would be feasible otherwise good luck to you.
4) In my testing, fairly long ago, I found that some browsers hadn't done a lot of the basics, which cuts down what you can use.
5) I've found some automated edited tools (I'm thinking online editors accessed over http here) completely mess up mailto: links. They don't understand much about them. (May not impact you.)
Long story short. I looked at it, a client was really keen on the idea. The area just stinks and would in reality cost you a fortune to do professionally (mainly in checking/unit testing).
The easy solution is to code your own. (With a modern web development environment it's trivial.)
Failing that I imagine somebody has provided a service to do just that. Maybe a service, to which you could POST data, using XMLHttp say, would be a clean solution. (I'd personally avoid anybody offering a service with GET strings in the URL.) Anybody here know of such a service?
When doing it I suggest not repeating the most common mistake. That's where you don't get a copy of your own email. It's almost standard that web mail forms don't do that. People sending the email won't forget that lack of manners.