Is GSM telephony feasible on the Samsung Galaxy Tab 8.9? I'm looking to buy a small tablet to use as a "table phone" for use with a headset, and for video telephony. GSM telephony is a must - I don't want to rely entirely on Skype, or Internet connectivity in general. (Plus I'm eligible for a landline number on my mobile contract so people can call me from normal phones for free, which is great.)

I am hovering between the 7" P1000 Samsung Galaxy Tab and the more expensive, newer 8.9" one. 

The 7" one officially supports GSM telephony. Various tests remarked that it's a bit too chunky for use as a normal mobile phone but for my use case on the desk, that doesn't matter. I really just want something I can carry around in a bag, and put on my desk to make calls with.

The 8.9" one supports GSM telephony, but doesn't come with it activated out of the box. You need third party Android products in order to use it.

This strikes me as odd, and makes me expect problems - why else would one build in the functionality, but not ship with it enabled? Telephony is the one thing I really, really need to just work - I can't afford to deal with dropping connections, bad voice quality, patches, and an overall "beta" experience.

Is telephony feasible on an 8.9" Samsung Galaxy Tab?
You write that the 8.9" tablet has support for GSM telephony but that it is not activated out of the box - and that suprises and concerns you.

Well, for many of us, the original 7" Galaxy Tab is the same: the hardware supports GSM telephony but the North American firmwares have no support for it.  Presumably this was done at the behest of the N.A. carriers who want users to use the tablet as a second cellular device - not as a replacement for a phone.

As a result, I had to root the Galaxy Tab and load a European firmware on it in order to activate its GSM telephony.  Once I did this it worked perfectly well as both a phone and a tablet.  (Though it does seem rather slow by today's standards, and it won't be getting ICS.)

It sounds like that is the same situation that you would face with the 8.9" tab: the fact that the GSM telephony was not supported out of the box did not mean that it didn't work well - but you need to be prepared to get your hands dirty to 'activate' this functionality yourself.