There have been rumors for some time. Personally, I think it's due for a replacement and has been for a while. Between 2006 and 2011, every Canon L-series prime lens of 100mm or shorter, except the 35L, was either updated or newly-released.
Having said that, even if they announce one tomorrow, it could be many months and multiple delays before it's actually available - so, I'll give my usual advice: if you want/need it now, buy it now. If history is a guide, recent updates (70-200 II, 24-70 II) the price of the updated lens will be significantly higher than the original, which means that after the MkII release retailers raise their prices on the MkI while they still have stock, and used prices go up accordingly. That happened with the 70-200 II, and I'm already seeing used prices of the 24-70 trending up. So if you get the 35L now, and do decide to upgrade, you'll likely at least break even.
Personally, I thought the 35L was due for replacement over a year ago...that didn't stop me from buying the current version 15 months ago, and I would have missed out on a lot of great shots if I had waited for a still-just-a-rumor replacement.