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 IIS/ASP.NET 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 XML based format on one site and import it into another. Is there something similar with wikis?
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.
The correct answer is ... "it depends".
It depends on which wiki you're using or planning to use. I've used various over the years MoinMoin was ok, used files rather than database, Ubuntu seem to like it. MediaWiki, everyone knows about and JAMWiki is a java clone(ish) of MediaWiki with the aim to be markup compatible with MediaWiki, both use databases and you can generally connect whichever database you want, JAMWiki is pre-configured to use an internal HSQLDB instance.
I recently converted about 80 pages from a MoinMoin wiki into JAMWiki pages and this was probably 90% handled by a tiny perl script I found somewhere (I'll provide a link if I can find it again). The other 10% was unfortunately a by-hand experience (they were of the utmost importance with them being recipies for the missus) ;-)
I also recently setup a Mediawiki instance for work and that took all of about 8 minutes to do. So that'd be my choice.
To answer your question I don't believe that there's such a standard as WikiML as Till called it.
As strange as it sounds, I've investigated screen scraping a wiki for a co-worker to help him port it to another wiki engine. It turned out that screen scraping would have been easier, quicker and more efficient to write to move this particular file based wiki to another one or a CMS.
Given the context that you wrote the question in I would bite the bullet now and pay the little extra for a windows hosted account and put Screwturn wiki on it. You're got the option of using file based or SQL Server based back end for it but because one of your requirements is low cost I'm guessing that you would use file based now for a cheaper hosted account and then you can always upscale the back end to SQL Server.