This is some speculation that MtGox does not have adequate BTC on hand to cover a significant volume of bitcoin wallet withdrawals.
By halting withdrawals it buys them time to (among other things) either; a.) increase their bitcoin balance, b.) come up with another excuse not to give customers access to their bitcoins, or c.) hope that this 2-day delay will somehow slow the bleeding once they re-enable withdrawals on Monday.
That is what most of us are worried about.
IF mtgox does declare that it cannot continue operations, does that mean all our money goes down with them? Or would they be professional enough to give us back what they owe us?
If they are in fact misleading customers, I wouldn't expect them to suddenly "do the right thing".
But if this is in fact a technical glitch, the balances are likely secure.