I'm no landscape expert, but I find that I usually end up using an 8 stop or 10 stop ND filter for long exposures. The SD1M can't shoot longer than 120 second exposures in its extended shutter mode and at I've found that 120 second exposures end up quite noisy, even at iso 100. No problems with 30-60 seconds though.
As for ND grad filters, I've stopped using them because the autobracketing on the SD1 (by up to 5 stops) is fantastic. Its more convenient and flexible to use layer masks during processing, if you have the patience to wait while the camera writes multiple long exposures to the card.
Here are 4 shots with the SD1M. The first three were taken with an 8 stop ND, the last with a 10 stop ND.