I use a Laney GS112 with mine and it works well, very reasonable price too (£99 at GAK).The 112 has a Celestion 70 Eighty speaker in it, plus they also have a 2x10 version with Jensons.
Bringing this topic back from the dead.Epiphone will soon be offering a matching cab for the Valve Junior.http://www.epiphone.com/default.asp?ProductID=283&CollectionID=13