Can't help you outside the US but had a recent domestic experience worth mentioning:

I've been an MCI customer for over ten years despite several interstate moves, and so I have MCI (getting NW/KLM miles) at home. This month I received an offer for 7,500 TWA miles to switch to Sprint. I switched and less than a week later MCI called me offering 6,000 miles to switch back!! So basically I got 6,000 miles to do nothing.

I don't know if other companies are as assertive as MCI (which has a dedicated "SwitchBack" department). By the way I had MCI on this current line for little over a year since I had just moved from another state. The MCI guy told me I had been a "valuable customer for 14 months" in his spiel, but I spend less than $25/month typically on long distance, and I don't think he knew how long I had actually been with MCI. He has a generous definition of "valued".

All these offers were the typical 1,000 miles per month over several months so long as you stay with that long distance company. And yes, they paid for switching my service to and fro.