No one loves the FA upgrade options at RF or 1B, and Orlando Hudson seems to be the only decent upgrade at 2B, and he might be pricey. Furthermore, there's a better than even chance that Rendon gets drafted and becomes our 3B by mid-2013, which makes you wonder if it's worth jerking Walker around the infield in order to win a couple more games in 2011 and 2012.
Hall is a career 3.9 UZR/150 at 3B; his career wOBA of .322 is middle of the pack for the hot corner. He is about to turn 31, so he could turn into a pumpkin, but the odds are probably for more of a modest decline. Hudson's got a better career wOBA (and better over the past 2-3 years), but he's about to be 33 - I don't think you can fairly project a big decline for one but not the other. Hudson's career UZR/150 is actually just 2.2 - he's been extremely variable - so, depending what you think Walker can accomplish with a full offseason/ST at 2B vs. moving back to 3B, it's not clear that there's a big defensive tradeoff either way.
I don't have a strong opinion, but it's an interesting idea to ponder. Do you want Hall or Hudson, and what would you pay for either?