With TextMate’s Markdown bundle, you can mark headers by moving your cursor to the line after the intended header, typing # and pressing <Tab>. The Textmate bundle insert #’s until the last # aligns with the header’s last character.

For example:

This Will Be My Header
#<Tab>

Turns into:

This Will Be My Header
######################

Here’s the equivalent feature in Vim:

:t.|s/./#/g

:t. copies the current line to the next line and s/./#/g substitutes any character on that line with a #. Use Vim’s built-in help command, e.g. :h :t, for more details.