I too have this issue since I bought my car in June 2010 (Santa Fe Premium, 7 seat manual transmission). The demo car also had the same issue. I reported the fault to the dealer on the day I picked up the car, they told me its a known issue and should be fixed with an ECU upgrade within a month, Well as I write this, its now been 6 months and I am also getting fed up with the fact I have a fault that Hyundai were aware of before I bought the car. I do not remmber the salesman saying "Thanks for the £23k you have just paid me and BTW there is a fault with your car, but we don't know how to fix it"
For any of you that have this issue, I have observed the following;-
Cruise will more often than not will not set when turned on. No matter wat you do, the cruise will not operate.
If you are lucky enough to get it to work, (this is normally within 1 minute of driving the car) at the first time of travelling in excess of 25mph (minimum cruise speed), set the cruise, it will set. I have to then hit set/resume immediately after every gear change. As long as I have the cruise set in this way , it will carry on working. If I deselect cruise for more than a few minutes, it will then not set/resume again until I stop the car and remove the keys for approx 5 minutes.
I spoke with Hyundai UK back in October and they did their normal PR bit and assured me that they were working on a fix.
I will give them 1 more month before I start to rattle a few cages. I do a lot of motorway driving and towing my caravan and this is one of the main feature I was looking for when I chose this model.
My suggestions to any of you is to call Hyundai UK and tell them you are not happy. I did it via email using their contact page.