As usual, I didn't have a clue how to answer her question about whether her SDL Trados Studio 2009 Freelance software could open a package created in Studio 2011 by her client. So I took the lazy but safe path and asked an SDL employee I trust (yes, there are such people believe it or not). He responded with the following comment:
"Answer to your question is yes... 2011 also has an option to create a package specifically for 2009 so you can go both ways quite easily... the creation of a specific package needs to be based on the receiving tool because of differences in the way projects are handled between the two, but the return package created is good for 2009 or 2011."
Not surprising that there is compatibility here, really, given that the new version also offers legacy compatibility with the old Trados Workbench RTF/Word bilingual format used by several other tools such as Wordfast Classic. Overall, it seems quite worthwhile for a user of Studio 2009 to upgrade, but there is no urgency to do so to remain compatible for working with others who have Studio 2011.