I am considering buying a Nokia 810. I have one question that the internet cannot seem to answer for me. When I have it connected to my cell phone via bluetooth, can I answer and make calls using it.

bluetooth lets you access files on the phone via the file manager on the n810 and use your phone as a modem to access the internet. it does not bring phone functionality to yout n810, though. (except voip, if that counts.)

if your question was meant the other way round (i'm not sure if i read it correctly): you can still use your phone for voice calls no matter what the n810 is currently accessing it for.

yep, your phone will remain active for incoming calls etc. it does drain the battery though

Yes and no. With umts/3g thats true, but with gsm/gprs/edge that can only happen if the data connection is idle.

So there is no way what so ever to make calls from it using a connection to your cell phone?

Well there is phonelink. But that just makes the tablet act as external numpad, the actual call still happens on the phone (tho with a bluetooth or wired handsfree, you do not need to get the actual phone out for the duration of the call).

phonelink, yes... in case your phone is supported.

seems to work with most phones in AT mode...

I have an LG phone, so if i use phonelink and get a bluetooth headset connected to my phone. The 810 is essentially is the phone then, correct?

It would probably be easier for the user (or at least more universally compatible) for someone to port the NoHands app. It would turn the N810 into an HFP handsfree device, allowing dialing to the phone, as well as serving as a "headset". I haven't tried NoHands yet, but it looks like what the OP was asking for, to me.
As for PhoneLink, that only works on non-crippled phones, so people in my situation (Telus, or similarly evil companies) likely won't be able to use it that way.