Sure, the allegations could be true, but - Birgitta Jonsdottir is a politician, and like everyone else has a price to do or say anything. It is within the realm of probability that she has been paid to take this highly public stance.
The fact is, a smear campaign is not just a simple accusation (in this case two coincidentally made on the same day), but rather a string of questionable accusations presented to people. Sure each one can easily be disproved in most cases, but if you make enough false allegations and pay a few people (especially those who appear to give credibility to an accusation) then these lies appear to be true.
Does ANYONE actually believe the USA can illegally invade another country, kill hundreds of thousands of people and manage to hide it.... yet would just stop at a simple rape allegation?? Uh, no! It has been widely covered that the US government is actively trying to destroy credibility of wikileaks, and sadly that will involve putting US operatives or paid rats inside wikileaks with the ultimate goal of taking this organization down.
The CIA has set up dozens of puppet governments in similar ways, so taking down an enemy website by 'framing' those who run it will happen. Truth be told, that "Collateral Murder" video makes much of what has happened look like a child's birthday party. For example, the blast and shockwave from the MOAB bomb can destroy about 8-10 blocks radius of a city, and we used these in Iraq... how many people who be disgusted if we saw the aftermath of just one?