Plugins: Vimballs already solve this problem largely.
Automatically in insert mode: Oh gosh no. The only case where you'd want to start in insert mode, I'd guess, is a new file. If you're opening an existing file, you're not going to start wanting to type at the beginning of it. Generally the first command in a file is a search or jump to line.
The author mentioned that he wanted a plugin that can quickly comment a line. There's more than a few ways to do this manually in vim that is quick. For example, to comment a block:
^<ctrl-v><n>jI# <ctrl-[>
where <n> is the number of lines to comment. You can also easily write a command in your vimrc to comment the current line in one keystroke.