Reread Momma L's post: These things DO happen. It happened to her family.
The risks are much worse than an arm or leg getting trapped. As someone else posted, the child can suffocate when he or she gets trapped between the crib side and the mattress. There are also strangulation risks.
Please go to the Consumer Product Safety Commission web site at www.cpsc.gov and search the words "drop side cribs." This is just the start of what you'll find there:
"As part of its commitment to ensure safe sleep for young children, the U.S. Consumer Product Safety Commission (CPSC) is once again warning parents and caregivers about deadly hazards with drop-side cribs. In the last five years, CPSC has announced 11 recalls involving more than 7 million drop-side cribs due to suffocation and strangulation hazards created by the drop side. CPSC staff is actively investigating several other crib manufacturers for potential drop-side hazards as part of a larger effort by the agency to rid the marketplace and homes of unsafe cribs. CPSC will continue to take aggressive action to address any risks and will keep the public informed.
CPSC staff has completed a comprehensive review of crib-related infant fatalities reported to the agency between January 2000 and the present. [2010] CPSC staff is aware of 32 infant and toddler suffocation and strangulation deaths and hundreds of incidents that were caused by or related to drop-side detachments in cribs made by various manufacturers."
Thirty-two deaths -- at least -- others may never have been reported to CPSC as being related to cribs. You say your only options are the pack and play or the borrowed crib, but you could also purchase an inflatable bed for him from www.onestepahead.com. These beds look like little life rafts; you don't use sheets on them (it would be unsafe) but they have a soft flocked surface. They were great when our daughter was small and we were traveling overseas and within the U.S. Yes, he will be able to move around -- it's like a toddler bed, not a crib, and is the equivalent of a mattress on the floor but with high sides so he can't roll off.
I think drop-side cribs are not worth the risk. It's only for one week, sure, but only takes one incident for a child to be injured or worse. And some people's argument that "My kids did fine in drop side cribs" is just silly; we all survived ratting around in our parents' cars with no seat belts but would we really put our kids in a car like that now? Nope. So why does "my kids were fine" get to be an argument in favor of continuing something that is dangerous?