I don't know how you found that but it's a good one! I verified your point by adding a completefunc argument to my code just to pass something (without modifying the SPServices 0.7.2 file). It works fine with a completefunc argument. However, when the completefunc argument is null or is not present at all (which is then null by default), I get the error in jQuery. Next, I removed the completefunc argument from my code and modified the SPServices 0.7.2 file as you mentioned. All worked well that way too. Oh, I also noticed that same command is at line 1211 so I modified it there as well.
Marc, I'm sure you'll come across this thread in due time and see what we're talking about here. It appears that jQuery 1.8.x is handling something different here than it did previously. Matthew's suggestion above appears to overcome the issue when users don't include a completefunc argument. I haven't experienced the issue with any other operations within SPServices using v0.7.2 with jQuery 1.8.x.
Now, all that being said, if I were practicing good coding protocol in the first place, I would have put something in the completefunc argument as a way to validate the results and would never have come across this issue. :P
Thanks!
Geoff