If the SIM card is on a contract (montly payment) then it can just be fitted into the modem and forgotten. Any charges you run up will just be billed on the contract, just like any contract mobile phone.
If the SIM card is a PAYG (pay as you go) one then you need to have a means to top up the balance without having to remove it from the ix104!! But the ix104 doesn't have a mobile phone in it, so you can't top up like you can with a mobile phone, by dialling a number and entering some codes on the keypad. Here in the UK, with both Vodafone (I used a Voda SIM) and Orange, with the demise of the topping-up scratch cards and their replacement with the swipe cards that have to be presented in a shop (UK only), things get a little difficult if one needs to do a top-up abroad...
The only reliable way to top up the card inside the ix104 is to register a credit card with the SIM card number, and then have a means of ordering a top-up from this credit card. The topup can then be easily done via the network provider's website, but the process to set up such an account involves them sending an SMS message to the SIM card (which they assume is in a normal phone) containing a security code and you need to enter this code into the website. So, you need to have a means of receiving this message. You can place the SIM card into a normal phone first and set up the website-based topup facility, and test it to make sure it works. With the software that comes with the Voda-badged PCMCIA GPRS card, this isn't necessary as the software supports SMS transmit/receive.
The process isn't foolproof though, as Voda's website topup was broken for several weeks early in 2005 and they never even realised. It's no wonder that serious data users use contract SIM cards.
This seems astonishingly stupid marketing. It could be an oversight, or it could be deliberate, to make it hard to use PAYG cards in fixed GSM installations like security alarms, where they will generate close to zero revenue for the provider because, in the absence of actual alarms being notified, the hardware needs only to transmit one SMS message every 6 months or so to keep the card account alive... I did find one online phone shop that was selling mobile topup blocks, where one merely had to specify the mobile number to top up, but it didn't work.
It is also highly desirable to disable content control with the GPRS network provider. The network provider occassionally incorrectly detects an "over 18" website access in the data, especially if the data is compressed using a compression proxy like Onspeed. What happens is that you click on some URL and nothing happens; after a long while the Voda page warning of an over-18 access appears. Such stupidity!