<p>I'm thinking of starting a wiki, probably on a low cost LAMP hosting account. I'd like the option of exporting my content later in case I want to run it on <code>IIS/ASP.NET</code> down the line. I know in the weblog world, there's an open standard called BlogML which will let you export your blog content to an <strong>XML</strong> based format on one site and import it into another. Is there something similar with wikis?</p>

I haven't heard of WikiML.
I think your biggest obstacle is gonna be converting one wiki markup to another. For example, some wikis use markdown (which is what Stack Overflow uses), others use another markup syntax (e.g. BBCode, ...), etc.. The bottom line is - assuming the contents are databased it's not impossible to export and parse it to make it "fit" in another system. It might just be a pain in the ass.
And if the contents are not databased, it's gonna be a royal pain in the ass. :D
Another solution would be to stay with the same system. I am not sure what the reason is for changing the technology later on. It's not like a growing project requires IIS/ASP.NET all of the sudden. (It might just be the other way around.) But for example, if you could stick with PHP for a while, you could also run that on IIS.