I would be also interested in this and also searched a bit. But it seems to me that every motherboard is manufactured in China or Taiwan.
Earlier today I read a news about ARM-based Raspberry Pi computer. After reading your thread I wondered where this is manufactured. On their homepage it says that their first intention was to manufacture it in Britain, but due to lack of suitable UK manufacturers and also because of following reason they manufacture it in Taiwan or China now (well for this low price, it would have been a surprise, if it were different).
I’d like to draw attention to one cost in particular that really created problems for us in Britain. Simply put, if we build the Raspberry Pi in Britain, we have to pay a lot more tax. If a British company imports components, it has to pay tax on those (and most components are not made in the UK). If, however, a completed device is made abroad and imported into the UK – with all of those components soldered onto it – it does not attract any import duty at all. This means that it’s really, really tax inefficient for an electronics company to do its manufacturing in Britain,[...]