Hi,
What you are intending to do I have just done. And I spent a while working out how much it it's going to cost. I have a Nokia 6230 with Bluetooth connected on a contract (Everyone 200) on T-Mobile. For a GPRS connection (I would recommend because a straight dialup will be less reliable and a lot slower), they charge .75 pence (not 75 pence) per K (1024 bytes). When I got the phone, it had Internet access set up on the phone itself.
First of all, when you use Tom Tom traffic, it gets it's traffic updates either by you specifying an interval automatically. Or by clicking a small icon to manually update.
I checked out how much data was being sent and received in each update. It worked out at about 7K. So that worked out at about 5.25 pence each time it updated. I called T-Mobile who arranged an allowance of 3 MB per month for £5. That would give you 428 updates a month. So it’s worth doing this if you intend to use a lot. Having used the Traffic software, I've found that for day to day driving, I prefer to just click the update icon. If I were to go on a long journey (for me that is about 1 and a half hours), I would set it to automatically update every 6 mins. This is a matter of preference at the end of the day.
Incidentally, it’s worth noting that if you were to set up a standard dial up via an ISP, you would probably have to pay a minimum connection charge plus time online. GPRS works out cheaper (even though I hate the idea of paying for data). Don’t know many other options at the moment.
If you need any help in setting up your PDA to use your Nokia via Bluetooth using GPRS, I’ll post the little info I got from T-Mobile. Your PDA is probably not setup to use GPRS by default (mine wasn’t).
In answer to your question about a mobile that will allow 2 Bluetooth at once, the answer is, “I don’t know of one.” Those who do know, please post as I’ve got the same predicament. I have even thought about buying a pay as you go sim card and putting in a cheap Bluetooth phone and just leaving in the car somewhere.
Another option is to buy a CF mobile phone card for your PDA. That would make the PDA a totally dedicated unit that wouldn’t rely on your mobile phone. But you will still need another sim card for GPRS usage. To see what I’m talking about, see here
http://www.infosyncworld.com/news/n/3542.html Search on Google for gprs cf card.
My experience of traffic so far (and I’ve only used it on 2 occasions) is that it looks pretty good once installed over Tom Tom. Today though, I was driving to Chichester on the A27 bypass from Portsmouth. Coming from the opposite direction there was a huge jam building up. The police stopped the traffic from going any further westerly at the Emsworth turn off (for those who might know this). I mentioned to my wife that they should really block the traffic at the Chichester end because this road is about 15 miles with no way of getting of. Anyway, about 2 hours later after spending time in Chichester, I decided to test the traffic software by calculating a route back to Portsmouth. As expected, Tom Tom routed my via the A27. I then updated the traffic information. Tom Tom reported no incidents, so I happily drove towards the A27. Guess what? The motorway was closed. So somebody somewhere failed to update this information for Tom Tom traffic users. Which is pretty bad considering this same information that Tom Tom get is also sent via TMC (over FM to your car radio).
It's also worth noting that traffic information can be updated via your home ADSL connection (if you have one). Either via your PDA cradle via your PC. Or via a WiFi point (if you have one). So you could update your traffic info before you leave the house.
Good luck
TomTom Traffic & Bluetooth 
Linear Mode
