I worked on this game for quite some time. Probably a good 20-30 days of work on it. This is my first ever large scale game and has a functioning high score system by rankz.armorbot. (went down long time ago)
I didn't perfect everything yet but I am working on it so criticism is welcome. I am more worried right now about coding problems and glitches than making the jumping and collision detection perfect.
I also used a template from flashvillage.com to create a little movie about the making of the flash game and what goes into creating something like this.
Well me and weirdlike a while back discussed something about platforming, and I have some pretty good working code, if you want to use some of it. (most of it is basic except for 1 part, where I was able to set the player's rotation to the angle of the ground he was on. e.g: the hill the player is on is 45 degree angle, so then the player's angle is set to 45 degrees, kinda like the fancy pants game).
I really suggest making your graphics in either illustrator or just even inside of Flash, that way it's very flexible and everything can be easily changed, plus no pixelation, since it'll be vector.
All I do really in flash is make platformers so if you ever need a little help with something i'm sure i have some code for it you could use.
One thing I do when I start a new project is I take the basic code of my last project and put it in the new one as a starting point so it's not so much work.