HTC might be having difficulty releasing one ROM for the HTC Hero, but one enterprising hacker has managed to install two at once on a rooted handset.
Kendon at the VillainROM forums has released a software bundle that allows you pick two versions of Android to boot alongside each other. You could run the stock Android 1.5, for example, alongside a less stable Android 2.1. All you need to do to switch between the two is reboot and then choose one, though the creator plans to introduce a version soon that allows you pick between ROMs by either holding or not holding the home button down when you press the power button to turn on the handset.
If you'd like to give it a go, you'll need at least some knowledge of the rooting process, and a computer running a flavour of Linux. Be aware also that the process takes some time to complete - around 15 minutes to "dualize" the handset, and slow boot-ups the first few times that you start your dual-booting handset.
If none of that has put you off, then grab the instructions and necessary files from the VillainROM forums. Oh, and remember that mucking with your Hero's internals voids the warranty and is entirely at your own risk.
Thanks to SoapyWeasel for the tip.