I initially wasn't sure if this was worth posting - if you have the same problem, you've probably come across the same links. But since I don't see a soup-to-nuts version, here goes.
Problem: you're migrating servers, and need to see what SSIS packages might be affected, so you need the connection strings for all of your SSIS packages. You could open up each package and check the connections - but that's... inefficient.
If you're storing the packages on disk, you could write a short (power)shell script that would scan the SSIS package files. But if it's in MSDB, you'd need to pull the package XML out of the system tables, then parse the XML. This is for 2008/R2 (and if you're not using SSISDB in 2012). In 2012, SSISDB holds the files differently, and I'm awaiting an answer from one of the MS people on the SSIS team (thanks to Jamie Thomson sqlblog.com/blogs/jamie_thomson/ for forwarding my tweet to him)
Code cribbed from somebody, possibly Feodor Georgiev (I found two links, below)
and the XML parse was a question I asked on StackOverflow. Thanks to Davmos for the help!