If it was possible to print notes that could not be easily detected as counterfeit -- millions would be printed and it would devalue the entire currency.
So bitcoin can't be devalued by "printing" fake "bills." That's a good prerequisite, but it's not a practical improvement for me over the current currencies.
Well, the value in bitcoin is that it can't be printed by anyone, even the goverments, uncontrollably - there will be only specific amount at max in circulation at specific time.
This might not be valuable to you, but it certainly will be for many else. Many people don't like the thing that goverments keep devaluing their fiat currencies.
A well-funded hostile government, bringing a lot of computing online aimed at "attacking" BitCoin would be a valid concern. I accept it's a tiny risk.
http://www.fin.gc.ca/budget06/bp/bpc3d-eng.asp names the amount for National Counterfeit Enforcement Strategy in 2006 as $9 million. I don't think these are the total expenses on preventing counterfeits, but it's within an order of magnitude. The official population count in the 2006 census was 31,612,897, so let's say the number of people who have paid any tax that year is around 30 million. Corporate taxes would complicate the "per taxpayer" calculation a little bit, but they would only make the amount smaller.
That's not a useful feature for me. Receiving counterfeit currency is very low on my list of concerns in life. It is a useful feature for governments, who would no longer need to ensure its currency isn't being counterfeited, but governments don't like this currency for a number of other reasons.