I've been following, but not contributing to, the discussion on twitter and sb (and elsewhere) about the question of postdoc salaries and taxation. And also the larger scienceblogosphere* PI-vs-postdoc chatter/discussion/war(?)
Obviously I have no firsthand experience in this matter but my completely unfounded opinion (a blog is a wonderful venue to express opinions for which one has no evidence or experience) is that one serious issue is not the actual monetary compensation postdocs get, or even whether it's taxable income or not, but the fact that at many (most? mine, anyway) institutions, they are classified neither as students nor as employees, which means that not only do they not get the benefits that employees are entitled to, they also don't have access to the services and support set up for students. Things like counseling services, housing and legal advice, occupational health and safety as well as general health services, etc. And many of them don't even have a contract covering their employment.
Most of this stuff doesn't matter most of the time, and changing it probably wouldn't have an impact on the general postdoc disgruntlement that exists, but I'm more worried about what happens to postdocs when things go wrong: when they get hurt, or sexually harassed, or otherwise need support. Many postdocs are non-natives to this country and don't have knowledge of or access to governmental services.
I wouldn't go so far as to call a postdoc "indentured servitude", but it definitely sucks. Overworked and underpaid, yes, but I think that the really crappy part postdocs is that they are undervalued. The PI can and should have an impact on this (in the form of having a contract and just generally not being a dick) but there also needs to be institutional recognition of postdocs as employees.
Science has changed significantly in many ways. One of them is that postdocs are now commonplace rather than rare. The fact that institutions on the whole have failed to adapt to this change is resulting in a general inequality and some of the "horror stories" we hear about.
I have more to say on the issue of whether there are Too Many grad students/postdocs, but I haven't sorted out just what that is yet.
* hey, shouldn't that be 'blogome'? Bwahahaha. Don't worry, I wouldn't do that to you. Except I did.