The new PV-Grub ability introduced by Amazon for EC2 has opened the door for more than just custom kernels, it also makes it a lot easier to turn VirtualBox and VMWare instances into EC2 instances. In the past I have written about transfering VirtualBox images to Xen but to do that with EC2 required a matching kernel exist for your VirtualBox installed OS that was blessed by Amazon. With PV-Grub as long as you can get a kernel for your existing system that is compatible with the EC2 infrastructure you can transfer it. A lot of the most popular distributions already have compatible kernels so that shouldn't be an issue and if you had to you could always compile the kernel by hand.
Some of what follows is exactly the same as my post about installing CentOS 5.5 on EC2 with the stock kernel. Once you get the hang of it you can install just about anything Linux based to EC2. I've broken this post into two parts to try and separate the generic transfer information from the specifics of an example. The first part that goes over the basics of what needs to be done to transfer any VirtualBox or VMWare box to EC2. The second part is an example of transferring an Ubuntu Server install to from VirtualBox to EC2.