The government thru any agency just like the IRS can get too creative to find ways to have some share of what we are earning. Tax evasion can be a serious charge and if we are earning some money via Bitcoin and we don't declare it then the government can act on that. This is exactly what the IRS is doing against Coinbase as IRS is demanding to have an access to the company's database to see those Americans who have never declared their Bitcoin.
No matter how we may proclaim that Bitcoin is anonymous and it is beyond the control of the government, the reality and truth is that the government can find a way most especially on taxing us. And making enemy with the government can be a terrifying thing. :(
The question everybody should really be asking is not "Could" but "When" will holders be attacked by the IRS and governments in general.
I would argue the reality is that Bitcoin is not yet but instead on the verge of being beyond the control of any government.
The last sentence of iamTom123's post - "making enemy with the government can be a terrifying thing" is the cause of most participants and observers denial of reality.
Bitcoin is in a natural state of war with the government because it is a competitor in the supply of money, a business/enterprise/function which is critical to the very nature of the modern nation-state concept. Currently governments consider it to be too trivial and more about a technological idea that can be subverted or hi-jacked for their own purposes and so they are likely not yet exerting significant resources in covert or overt ways to attack. But as it grows the nation state stance is likely to change suddenly.
I believe the answer to "when" is somewhere around another 10x higher price / market cap jump for bitcoin and crypto in general. Best to prepare now for an eventual conflict instead of being scared about it. Otherwise don't get involved in bitcoin.