I recontracted with Singtel a couple of months ago for a BB Curve 9360 3G Flexi Lite + BIS plan. But I now want a S3 or a Nexus. I plan to get either one from a third-party source (ex eBay). Will Singtel mind if I use my simcard with the current line contract on a different phone?
I think it may affect your billing because the terms are very very specific to Blackberry services:
You can see the FAQ and the terms in the links given in this link:
Basically, call Singtel up and leave no stone unturned.
I dunno what your BB plan involves. But if you currently using BB services on your sim card, you should be aware it might not work on a non BB phone. That's all you need to know, singtel won't care where you stick your sim card, as long as you pay what they bill you for.