I discovered the following solution after finding out that you can send other GSM generic codes (dont know if that's the right word for it) like *t# (and a space before the actual message), which will return you a SMS upon succesful delivery (nice if you're not sure wether the whole SMS through Bluetooth actually works). You can do that with messages from your phone as well, and I believe these codes are the same on every GSM-net in the world.
When you recieve a long SMS from a friend, and you are connected to the Address Book over Bluetooth, two messages pop up from the Address Book with some gibberish code in the beginning of each message. The code I got from the first pop-up is é@ø¿!, and the next pop-up was é@ø¿!¡@. As I said, I believe these codes are the same all over the world, but you can test by sending yourself a long message and looking at the code.
If you the write the first code in the start of the "first" message, and the second code in the start of the next, they will be recieved as one long message on the reciever's end upon delivery of the second SMS. This also means that you can't just use the code if the message is under the normal 160 characters, as it will not be sent at all.
There's probably a third code if the SMS spans across three messages, but I haven't tested for that yet.
[robg adds: Any typos in the above codes are probably my fault; I can't test this one, though, so someone will have to let me know if they work as described or not.]

