What Mhash would I need to earn 1 BTC a day ?

It depends on where the difficulty is.

Today's Total Network hash rate is : about 25 TH/s

Difficulty is: 3,249,550

So about 7 GH/s would net you about 1 BTC per day.

However, once ASIC's are delivered in a week or three, the hash rate will go up.

If Avalon delivers next week they will be adding about 30 TH, some of the GPU will fall off. So lets assume the total Network has rate doubles, then the difficulty will also double. That means it will take double the hashing power. So it will take 14 GH/s to make 1 BTC.

Butterfly labs is estimated to be delivering about 300 TH to the network. So if you figure 300 for BFL and 30 for Avalon and then figure many more ASIC orders will pour in, a safe next estimate will be at 500 TH. since that's 20 times the current hash rate, you will need 140 GH to earn 1 BTC per day.

Another way to look at it this there is a fixed rate of BTC that enter the market per day

25 BTC per Block * 6 blocks per hour * 24 hours per day = 3600 BTC per day.

So you would need 1/3,600 of the current average hash rate to earn 1 BTC per day.

Remember that the average hash rate for the previous two weeks, which is what determines the current difficulty.

The difficult changes every 2016 blocks. The difficulty is used to steady the rate of Blocks found to about 1 every 10 minutes. So that works out to about a change in difficulty every 14 days.