That work needs to get done. It is your managers job to see that it gets done. Assuming you ascend to that level in your career you will have to make those choices too. And yes with Great power comes Great Responsibility, but it does have some perks like picking the projects you want to work on. In the end if the work needs to get done and it doesnt get done he will also probably be the one staying late and helping to make sure it gets done or fixed. Take that he trusts you to get it done right so that doesnt happen as a positive.
Managers need to develop their skills too. Your manager needs to develop his skill set to continue to grow. The project may be one he feels comfortable starting out with. He needs to learn for himself what works what doesnt so that he can mentor others. A good manager will also ask for help if they need it. Instead of asking to join the project ask if there is anything you can do to help. Let him know you have some experience in Flex(if he doesnt know already) and that you would be glad to help out any way you can.
Do those "Grunt" projects to the best of your abilities. Show him you are a team player and can be counted on. Your manager will reward you eventually. You may not even know that he has done it already. A good programmer that is a bad employee is not what a business needs. Be a good employee and a good programmer. Then those projects will come. But you will never get them all.