Returns are not very common in Europe (like they are in NA). Most stores will charge a restocking fee or even not take the item back at all.
Now in NA returns are common but I don't think there will be a huge return of one console compared to the others. Maybe the exception is the Wii since some people might just sell it for a profit and so less returns.
DS, PSP, PS3 and 360 should all see equal return percentages (unless lots of 360's RRoD in the next few weeks, j/k)
More interesting will be to watch the end of year adjustments. We know PS3/360 and Wii are a little over tracked in US/CAnada compared to NDP so it will be interesting to see how it gets adjusted when MS, Nintendo and Sony release their numbers.







