Yesterday I created a local installation of .Text - this will enable me to perform client tests without constantly creating test posts that I then have to try to find and remove.
This has already borne fruit - I was able to determine that the problem I had with category posts using SimpleBlogService.asx was specific to this site, not an intrinsic bug in .Text. That's pretty much what I thought.
I've also been trying out the AspNetWebLog web service as an alternative for posting - there are no problems with categories, and it's possible to retrieve a list of entries and so on. The only thing that's a bit iffy is that the web service is marked as a beta in the description, although that's possibly out of date. There's also the possibility that someone might have a blog running on an earlier version of .Text that includes SimpleBlogService.asmx but not AspNetWebLog.asmx. I might go for an initial call to AspNetWebLog.asmx with a fall back on SimpleBlogService.asmx if that's unsuccessful. Icky but it's one approach.
This might also be the time to tackle the provider issue, providing I can do so without too much of a negative impact on performance.
Update
I've clarified my thinking on the provider issue: providers deliver extensibility whereas in this particular case I'm really just interested in a degree of flexibility. I can't see how I'd use the provider pattern without using Reflection, and I don't think I could justify the performance hit for the sake of providing for extensibility that would be more theoretical than actual.
Further update (Tuesday, September 7)
I've just been playing with the Blogger API, as part of my ongoing efforts to see if I'll be able to write a client that handles Blogger and blogs that use the MetaWebLogAPI (in other words, XML-RPC stuff). I'd considered seeing how much of Charles Cook's excellent XML-RPC.NET library can be ported to the Compact Framework (BTW most of it, as far as I can see), but I'm avoiding that option for now because I'm going to keep things lightweight if I can, and don't need to support a lot of different functions, so at the moment I'm experimenting with just working with raw XML streams. I just managed to get a list of my blogs from the Blogger API (I only have one Blogger blog, and it has no posts for now), so I'm quite pleased. This bodes well considering the similarity between the Blogger API and MetaWebLogAPI, so watch this space.
(Even if this pans out I'll continue to use the web service interface to .Text blogs, since I think soap-based web services are where people should be going in future, not XML-RPC. I realise all the action in Blog-land is currently with XML-RPC, but I personally think that's short-sighted)