Data Plan vs. Smart Phone Data Plan?

My small, local carrier, Cincinnati Bell Wireless (Welcome to Cincinnati Bell) has given me a geat deal on a data plan for years now: $15/month for unlimited data and texting. I use roughly 20MB/month on my Nokia 6300. They're a GSM carrier who I hear actually leases network from AT&T.

I'd now like to upgrade to something like a Nokia N95 or Nokia 5800. However, they say this requires a "Smart Phone Data Plan" which is roughly $30/month.

My question is: what's the technical difference? They told me that because a Smart Phone is "always connected", it costs more. But there's nothing stopping me from using a problem on my phone now that does data transmission non-stop (right now I use Google Mail app, Opera Mini, Weather app, RSS Reader app, Chat app, etc.). Are they just trying to leach more money out of me, or is there truly something that would prevent me from using, say, a self-purchased unlocked Nokia 5800 with my current SIM card and leverage my existing data plan as is?

 

For what its worth, I did buy the phone outright (Nokia 5800... through Dell... fully unlocked) and slipped my SIM card in. It works! That said, they do see that my phone is the new model, but no one has done anything about it yet.

Now I say it works, but I'm not sure if its fully working. I can certainly make calls and browse the web. However, during the settings push from Cincinnati Bell Wireless, it created two access points: an "Internet" access point (packet data, web.gocbw.com) and a "WAP services" access point (packet data, wap.gocbw.com). The former never works and the latter always does.

I also seem to only be able to have one connection open at a time. I could only run one program at a time on my old phone, so I don't know if this is a limitation of CBW, GSM or the "dumb" data plan.