>> In this situation, is this an impeachable offense?

http://en.wikipedia.org/wiki/Impeachment#Impeachable_offenses

I like Gerald Ford's definition of an impeachable offense. He was an astute lawyer and political leader. He knew the workings of the House as well as anyone before or since.
In that context you can call Obama's handwriting an impeachable offense if you can get enough votes behind you.

Historically speaking the impeachers have done more political damage to themselves than to the impeached.
All of the folderol starts to look pretty silly while real issues are being ignored.
There were some substantial efforts by house dems to impeach George W Bush for whatever and Nancy Pelosi shot them down in a hurry. She had seen what the repubs did to themselves with the Clinton fiasco.