On the other side of it, I'm not sure your concerns that false partisan attacks will win the day are justified - look at the "birthers" who have been thoroughly discredited and marginalized (ie the ones who claim Obama wasn't born on American soil).
At least with respect to Acorn, it has had a slew of scandals plague them and yet various levels of the US government had continued to trust them with rather important tasks - e.g. help at the IRS, census, etc.. In my view these were far worse offenses than the 'entrapment' that brought them down. There is no reason that larger media outlets shouldn't have pursued this story themselves - after all - the cost even of putting out the Acorn story was less than five thousand dollars. Meanwhile, supposedly reputable outlets like the NYT had trouble covering the story until the Obama Administration and Congress began to react and cut funding and ties to Acorn.
I'm all for disclosure. Let's be open to criticism/investigations from anyone and everyone and let them be judged on their merits.