When I bought my N810 back in Feb of 08, I also bought a 6G Sandisk micro SDHC (class 4) from Amazon. Upon discovering that the N810 doesn't actually take micro cards, I bought one of those Sandisk adapter sets (micro - full, and micro-mini, and comes with a 1G card) from Radio Shack (I was deparate... what can I say).
I have had almost no trouble from it over the last year and a bit.
I say almost, because every now and then, I have to re-insert a couple of times to get it to recognize. I wonder if the previous comment (about inserting too hard is the culprit). I find that using the stylus to press on the mini-adapter is the best way to get it clicked-in correctly (I have fat fingers, so I generally can't get it to click by myself).

Oddly, as I was reading this thread, I thought I would see if there were any identifying marks on the adapter that might narrow down the problem. The only thing I found on my adapter was a date-stamp on the back (says 2007-03-08A). But, when I put the adapter back in, I got my first ever 'Corrupted' error.
So, a reinsert. Still the same error. OK, a reboot. Problem fixed. Memory card is used and working again.

Also, I have noticed a few times in the last year that an improperly inserted card may also cause the internal card not be recognized. As soon as the external card is reseated 'correctly', they both magically appear.

My guess? Something about the N810s funky slot does not play well with Sandisks adapter. Having said that, they can be made to work, as I have done so successfully for over a year.