Barring a loose cable, the biggest worry about the BICEP signal is that the collaboration may have underestimated the galactic foreground emission. BICEP2 performed the observations at only one frequency of 150 GHz which is very well suited to study the CMB, but less so for polarized dust or synchrotron emission. As for the latter, more can be learned by going to higher frequencies, while combining maps at different frequencies allows one to separate the galactic and the CMB component. Although the patch of the sky studied by BICEP is well away from the galactic plane, the recently published 353 GHz polarized map from Planck demonstrates that there may be significant emission from these parts of the sky (in that paper the BICEP patch is conveniently masked, so one cannot draw any quantitative conclusions). Once the dust from the BICEP announcement had settled, all eyes were thus on precision measurements of the galactic foreground. The rumors that have been arriving from the Planck camp were not encouraging, as they were not able to confirm the primordial B-mode signal. It seems that experts now put a finger on what exactly went wrong in BICEP.
To estimate polarized emission from the galactic dust, BICEP digitized an unpublished 353 GHz map shown by the Planck collaboration at a conference. However, it seems they misinterpreted the Planck results: that map shows the polarization fraction for all foregrounds, not for the galactic dust only (see the "not CIB subtracted" caveat in the slide). Once you correct for that and rescale the Planck results appropriately, some experts claim that the polarized galactic dust emission can account for most of the BICEP signal. The rumor is that the BICEP team has now admitted to the mistake [Update: this last sentence is disputed].
I cannot believe this; one of the most important discoveries in recent years depends on a pdf file and it is unclear what it actually shows? Can the two teams please talk to each other and exchange the necessary data?
Jester was the first to report this story, but I think there is more than one jester involved in this ...
UPDATE: Peter Woit of NEW has more details.
The dust hasn't settled on this one yet, but it looks like the BICEP2 team can put Grandma's picture back in that spot on the shelf they had saved for the Nobel Prize, for the moment.