When you bluejack, does the victim's phone/PDA just show that there is an incoming contact or does it get accepted by the phone or PDA and entered into the address book?
On most models the phone will bleep the same tone as when a text message is received, and it will show a message similar to "name of contact has just been received by Bluetooth". If the user then accepts it, the contact will appear in their phonebook. From here the user can see what is written in the other fields too.

I want to know how to send a business card, if at all possible, from a SE T610.
A simple answer would be that a business card is the same as a phone book contact, so yes, you can send them from a T610 because it has Bluetooth. You can follow the same guide that I have written for sending contacts.

Whenever I try and bluejack someone it asks for a passkey. What am I doing wrong?
Ah. You are trying to pair your phone to another Bluetooth device. This is not what you want to do and won't work. Instead of going into 'My devices' in your Bluetooth menu, select the contact/item you want to send and choose 'Send' then 'Bluetooth'.

Is bluejacking free?
Yes. It's free because it uses technology on your phone not your simcard. Since your network only has control on network-provided features, and Bluetooth is a feature on your phone not provided by your network, it is free.

Will my phone number be sent out with anything I send when I bluejack somebody?
No. As I said earlier, Bluetooth is a feature on your phone and has nothing to do with your network and therefore nothing to do with your phone number.

How do I stop people from bluejacking me?
You can either go into your Bluetooth menu and switch it off, or if you need Bluetooth on for one reason or another you can simply set your phone to 'Undiscoverable' in your Bluetooth menu. This means that other Bluetooth devices won't find you when they search for Bluetooth devices.

Can someone hack my phone and steal my phonebook contacts?
What you are describing is a form of 'bluesnarfing'. Please read this article which explains bluesnarfing in more detail