I only have one computer (running OS X), and need to test in both Safari 3 and Safari 4. While Safari 4 is the "official" release, I don't believe it has been pushed in Software Update yet, and therefore probably hasn't been adopted by most users. Even if it has been pushed, it's still a safe bet that many users haven't updated.
The problem is that updating to Safari 4 replaces the system Webkit frameworks, making it difficult to run it at the same time as an older version.
One solution that occurred to me was to download a Webkit build from a few weeks before the release of Safari 3.2.1 and assume it's relatively the same as the release, but I'd rather have the "real thing" to be safe.
Anyone have any good solutions to this outside of having multiple computers/operating systems running?