On 850 T5 recently acquired, I notice unmistakable antifreeze odor inside as soon as engine is warming up. Noticeable right out of floor and vent ducts. The vent air is clear and smell is not present for 2 minutes or so after cold start-up when engine is still cold. I know this sounds like the heater core, but in other vehicles with heater core failures there was fog and an oily deposit on the inside windows, which I do not notice.
Are heater cores a common failure item? and/or are there other possible causes?

It is a heater core for sure and yes it is a common failure on these cars. What you will find is probably happening is that the core is leaking just enough to get a small amount of coolant on the outside of the core. When you start it up it heats up the core which causes two things to happen: first it causes the leaky core to heat up, expand and seal and two it causes the coolant on the outside to evaporate. So, you smell the coolant for the first few minutes during warm up and then it goes away. In my experience this is usually a fairly small window before the core takes the complete and total dump. You should go ahead and take advantage of this window and get it repaired before it takes the complete and total dump and leaves you stranded.

Thanks to Rule 308 for the reply. Do you or any others have any suggestions for the replacement? Is it a difficult job for a service shop or can I attempt it (access?)? and where is the best source for the core?

As for your first question, Yes, this is a very easy repair. At least it was on my 1995 850. You can see the procedure illustrated at www.volvospeed.com. The exact link is:

http://www.volvospeed.com/vs_forum/index.php?showtopic=9717&hl=Jeater

I just replaced mine around Thanksgiving and I did not have to remove the glove compartment as shown in this procedure.

I purchased my heater core from FCP Groton for $128. I could not find one locally and all of the other online parts sites listed it for $280. It has been working great (9 degrees when I went to work this morning).

Good Luck