Definitely worth checking out with a tax advisor, as different cantons have different rules, but from my understanding the situation is as follows:
* all income should be declared, as should your global wealth. These are taxed separately. Tax on fortune is really small (unless you are really, really wealthy - and they you should definitely be seeking professional advice )
* Your income from movable assets (salary, dividends, etc) will all be put together to be taxed, irrespective of where it has come from (i.e. irrespective of which country it has been earned in) - however, you can also deduct tax paid, loan interest etc, etc.
* Your income from immovable assets (property) is not taxed per se, but is taken into account when working out your tax rate. The tax rate you pay on your overall income goes up depending on how much you earn, so if you earn for example 50,000 CHF per annum you could be on 10% tax on that whole 50,000 CHF (these are made up figures by the way just to provide an example!). If you earned 75,000 CHF, your tax rate on the whole amount (not just the 25,000 over 50,000) could go up to say 20% (again - fictitious rates, but do note that the tax rate is not linear - it seems to go up exponentially after a while!). Again, you can discount things like mortgage interest, agent fees, repairs, etc.
So, if in my fictitious example you earned 50,000 in a mixture of wages and dividends at a tax rate of 10%, but then earned an extra 25,000 from property overseas, you would end up paying 20% tax on your 50,000 CHF instead of 10% - so it is really a stealth tax even though you would not be officially "taxed" on your 25,000 CHF of property income.
With regard to past tax returns, I had no idea about this when I got to Switzerland (was being taxed at source) and only found out after two years that if your assets are over a certain amount (not that high if you own property) you should actually do a normal tax return - at least in Geneva, so we got an accountant to help us do backdated tax returns for 2008 and 2009. The GVA tax authorities were all ok with it - I think they are generally fine if a genuine mistake that someone is trying to rectify.