No announcement by poloniex but steem deposits seem to have been disabled at the moment.
Consequences
As Poloniex is the main place STEEM are traded this means that no new STEEM can get on the site to be sold but other assets can still be traded for STEEM, this will lead to the price rising. When deposits are allowed through this could also lead to a temporary crash.
Causes
As said poloniex have given no reason but I made a transfer to them before they shut down deposits last night at 20:00 UTC and it seemed to correspond to steemit loading very slowly, so maybe poloniex's reasons are due to this. Even this morning steemit seems to be loading poorly, if I open a new tab of steemit I am no longer logged in. It is possible if the automated program poloniex use to access steemit is having trouble that they would temporarily disable the deposits.
Where can I trade now
Bittrex is the second biggest exchange for STEEM trading and seem to be accepting deposits still.
Update 1
Interesting, I just noticed that poloniex are still accepting SBD (Steem dollars) so that would imply that the system is still fine. Or maybe as there is not much traffic on the SBD/btc pair that it didn't get flagged by their system.
Update 2
still disable and no communication from polo, also looks like steem dollars are disabled now
Update 3
From @ashaman's comment below
Nobody panic. Steem is hardforking in 35 minutes. Poloniex disabled deposits in preparation for the hardfork. In all likelihood, they have already upgraded their witness_node, and are currently testing the new version to make sure no bugs in relation to their backend were introduced. Deposits and withdraws of XMR also get disabled in anticipation of a scheduled hardfork, and are reenabled after - since a hardfork carries a risk of a chainsplit (however small), to avoid the (however unlikely) possibility of double spends exchanges will disabled deposits/withdrawals of a coin when it is forking. Steem and SBD are also disabled with OpenLedger right now. Just sit tight, everything should be back to normal in a few hours.