Earlier today, the Lions signed kicker Dave Rayner and released cornerback Jonathan Wade. Those weren't the only moves they made today, though. Later in the day, the Lions released center and claimed quarterback off waivers.
Wallace was signed to Detroit's practice squad in September and was released a week and a half later. He then signed with the Jets' practice squad about 10 days later. 10 days after that move, he was signed to the Lions' active roster. Detroit released him today, right after playing the Jets, which I'm sure was no coincidence. Players signed from another team's practice squad do have to spend three weeks on the 53-man roster, so that's why he was released now, but I'm sure it was no coincidence that they signed him when they did.
Robinson is a rookie from Oklahoma State. He was drafted by the Patriots in the seventh round and was signed to the Seahawks' practice squad after the final round of roster cuts. He was activated to Seattle's 53-man roster last week to serve as the backup quarterback with Matt Hasselbeck out. Seattle released him, likely with the hope of sneaking him through waivers and back onto their practice squad. The Lions put in a claim for him, though, meaning he will be headed to Detroit rather than staying in Seattle.
What this shows to me is that Matthew Stafford will not be placed on injured reserve, at least not yet. As far as I know, Wallace was released to make room for Robinson. Unless the Lions made another move that hasn't been reported yet, Robinson has joined the team as the fourth quarterback on the roster, so Stafford hasn't been shelved just yet.
This move also suggests that Shaun Hill will not be ready for Sunday's game. If Hill were healthy enough to play, the Lions wouldn't necessarily need to add another quarterback to the team. By claiming Robinson, however, Drew Stanton would have a backup in case he goes down. Obviously Robinson will only be able to learn so much of the playbook before Sunday, but the Lions need a healthy backup, which is likely why this move was made.