Thanks for the replies, all. OK, so the pre/post-tax status of what's left behind in the tIRA after a partial conversion does not change. I'm not yet sure I have my head wrapped around all the complexities, so let me ask about a second example that appears to yield a double-tax trap.
In my initial example, the tIRA had $5000 PRE-tax dollars and in 2010 the person added a $5000 NON-taxable contrib. For the second example, let's say instead in 2010 the person adds a $5000 TAXABLE tIRA contrib, making for $10000 total in tIRA, and then he backdoor converts $5000 to Roth. Come the following year's April 15th, he'll owe tax on the $5000 tIRA contrib, which makes half his tIRA pre-tax dollars and half post-tax. Because his tIRA is half pre-tax dollars, half of his $5000 conversion to Roth will be subject to tax. So, not only will he be taxed on the initial $5000 tIRA contrib, he'll be taxed also on 50% of the conversion! If he repeats this in 2011 and later, the same double-tax will occur each year!
Is that correct? It sounds strange, so perhaps I'm misunderstanding some part. If it is indeed correct, then the double-tax can be avoided by converting all tIRA dollars to Roth, in which case the following year's tIRA will be taxed but the backdoor conversion will not.