I've had a look at the IFS report, which is what all the news reports are being based on and from what I understand there is some massive scaremongering going on (probably because most journalists don't understand Tax Credits). This is my understanding (please note I cannot predict what will actually happen in the budget!)
It might help to briefly explain how tax credits are calculated. Basically, there are a number of amounts of money called 'elements' (working over 16 hours a week, working over 30 hours a week, couple and lone parent element, the amount spent on childcare up to a certain amount, a family element and then an amount for each child) which are added together to give a total theoretical entitlement to working and child tax credits. This amount is then gradually reduced based on the household income. The first £6420 of income is ignored and then for every £1 that you earn above this, 39p is taken off your tax credits entitlement.
Once the amount of tax credits has been reduced to £545 it is currently frozen at this amount until your income reaches £50K. So, in practice if you only have 1 child then anyone on an income between about £24 and £50K will get £545 a year.
For anyone with an income of above £50K the current system dictates that they have their family element (the £545) reduced by 6p for every £1 they earn over £50K. So, you will continue to receive something until your income reaches about £59K.
The proposals about 'no child tax credits if you earn over £25K' which we keep reading about in the press are actually that this £50,000 threshold at which you are guaranteed to receive at least £545 is removed completely. Because of 39p being removed for every £1 you earn, in practice this would mean that if you have 1 child then you would not receive anything in tax credits once your income reached about £25,000. If you have more than 1 child then you would carry on getting some money until your income reached a higher level (about £31K for 2 kids, £37K for 3 kids etc etc).
The jist of all this in case you didn't follow this (hideously complicated system) - if you currently get more than £545 a year in tax credits (£1090 if you have a baby under 1 year) then you shouldn't be affected if these proposals are put into practice. If you currently get £545 or less then you may get a reduced amount or nothing.
Hopefully this should reassure people a bit. Certainly don't go giving up your job or reducing your hours - tax credits are currently are likely to continue to be reduced on a gradual basis and you should never be in a situation where you will be better off overall by earning less money at work, at least in terms of tax credits.