I have had to replace the suspension bearings (8 of them) on my 2005 Reign three freaking times!!! The first time was my fault for spraying degreaser on them when I thought it was for lubing. The second time, the LBS thought they would save me money and only replace the ones that were bad. Well that didn't turn out too well since it wasn't long before the other ones go. You are supposed to replace them all. Check the casing where the bearings go. If they have been worn out the bearings no longer seat in properly. Some bearings have to be pressed on and pressed off so if the mechanic is not careful, they will mark the casings. The last time I replace them, the LBS told me that if it happen again, they would send my frame to Giant to have it inspected and see what they could do about it since the bearings should be lasting longer. It could be a slight bent or something. Hopefully, you don't have similar problems.
I wanted to replace the Giant bearsing with aftermaket ones made by Enduro. The LBS adviced me not. I don't know if they wanted me to keep going back or if they just didn't really trust the Enduro bearings. Other swear by them. Look into them.
Also, check the mtbr.com forums for the type of bike you have to see if others have similar problem and how they fixed it.
If you tighten them yourself like Scott, make sure you use the manufacturers recommended pressure. Do NOT over tighten or under. Another reason why mine may have gone bad.