The Sharpie Liquid Pencil writes like a pen but can be erased like a pencil, and it becomes as permanent as a permanent marker in three days.
The secret to the Sharpie Liquid Pencil is that it contains liquid graphite, which enables you to write as cleanly and smoothly as with a pen. However, if you have second thoughts, you can erase the “ink” just as you would if your words were written in pencil. But beware, you only get three days to do so, before the ink becomes permanent.
According to the Sharpie blog the Liquid Pencil will be available in September, 2010, and Office Depot has a version available now for $4.99, which includes two pens and four erasers.