Victor came across this fix tonight and, so far, it appears to be working. I won't know for a couple of days, yet, if this solves the problem, but it seems like we are on the right track.
This is a very low-level, scary, nasty, don't do this unless you know what you are doing fix, but I wanted to place it here in case anyone else is experiencing similar problems.
The Fix for securityd Eatings Gobs of Ram When Updating Keychain Entries
A few nights ago I was updating some not-to-be-named software on my laptop. This piece of software had a few passwords stored in the Keychain. Since said application was recently updated and therefore the code was modified, the system asked me if I wanted to give access to the keychain to this updated application. The dialog that it shown to the user is shown below:
Bad things happened when I clicked the "Change All" button to once again allow this updated application to access all the passwords it was allowed to access. Specifically, the securityd process was using 1.3-1.7GBs of ram (the rprvt value is all that matters). This was really, really bad as it caused my machine to page-out and page-in like crazy. Due to the high memory usage, it also caused my boot volume to run out of space because of all of the swap files in /var/vm/. My point is that very, very, very bad things happened. After I cleared a lot of unused crap (Garage Band loops and old iDVD themes) off my boot volume, I rebooted. I then tried launching the updated application again. I got the same dialog and the same problem. However, since I now had enough hard drive space available, I just waited for about 10 minutes. The passwords were accessed successfully. I then relaunched the application and securityd crashed. Lovely. Rebooting just repeated the cycle. Also lovely.