What I mean is exactly like what hunnaryb said.
In other words, the imagined scenario as follows:
Some websites offer instant Bitcoin purchase like Truecoin. If such website is embedded inside any merchant's website using HTML tags like
First of all, I don't think there would be demand for such service. Most merchants (even if they are BTC enthusiasts) would prefer to keep most of their income in fiat, simply because they pay with fiat for vast majority of their operational expenses (stock, electricity, salaries etc). I can't imagine why any merchant would opt for instant conversion to BTC of all their sales income (well, maybe it would work if you operate in a country with hyperinflation).
The other thing is the risk of chargebacks. It's easy and safe to instantly convert fiat to bitcoins (i.e. as BitPay does it) as BTC payments are irreversible, but it would get more complicated if you try to do it the other way around.
But yes, technically speaking, it's possible.