I think the reason he dropped down to Southampton was that, he could see Scunthorpe were going to struggle in the championship (which they eventually did) and with the money Soton had (biggest budget in L1 at the time if i remember correctly plus had a great squad including Rickie Lambert) it was an easy promotion to add to the CV.
I don't think we'll see Nigel Adkins in League 1 any time soon.....