It is most defiantly a DDoS, you can tell they are being hit if you have the MtGox ticker chrome extension. The second it goes gray you know MtGox is under attack. You will however be able to "buy" bitcoins through your account page but you will be getting fuct over if you do, as you will be put into a buy queue at a high price (if you thought it was good idea to buy at say $190) you will be locked into this queue and no matter how many times you click cancel your pending order will not cancel, and even if it does their db is so fuct that even when the pending transaction is gone from your account page the transaction will stil go through the second the DDoS is stopped, This happened last week as well when the price was at $112 then shot up to $136, it had then hung around $126 for about a good 2 hours and then the DDoS commenced and dropped the price back down to $102 after that DDoS stopped price shot back up to $119 and took another 3 days to hit $124+ mark. within 3 days after that price has been over $200
So you can see how easy it is to destabilize the bitcoin market quite easily...
Funny enough if you make a bot using the shitty MtGox api you can avoid the DDoS entirely to make your trades and this is how they are snatching up all the coins and setting the market during the DDoS. You can however sell your coins through your account page and it will queue them for sale without any issue, however just like the buy issue trying to cancel during the DDoS is impossible, so you see how a bot comes in handy ;)
Also MtGox is total trash for claiming to have DDoS protection they seem to go down in flames pretty damn fast, so until more trading sites come online you are pretty much stuck with the morons running MtGox. They let the DDoS happen because they still make $ of every transaction either way so why should they give a shit whats getting traded at any given time... It would be easy enough for them to just shut down for a few hours but you know they wont.