Q: my ipad 2 3g keeps coming up with "could not connect to cellular data network"

I too have a iPad and understand how important it is to have access to the internet. I found a peace of information and copy pasted it underneath that might be of help.  If the information underneath isn't exactly what your looking for, try this link  http://support.apple.com/kb/HT1970.  

If you insert a new SIM or micro-SIM card into your iPhone or iPad, you'll need to download the carrier settings for the new carrier wirelessly. Carrier settings are available only for iPhone support carriers and iPad partner carriers. If you're not using a partner carrier, you will need to set up the carrier's settings manually. Refer to your carrier for appropriate settings.