Any truth to the rumors about fatigue cracking aluminum baffles from the vibration on the ultrasonic?
Not that I know of, but there have been concerns voiced. I think if they have any merit, it'll be highly dependent on the kind of aluminum alloy being used.
Edit: Just to clarify, you absolutely shouldn't clean aluminum components in an ultrasonic bath (and other soft metals might warrant care too). Ultrasonic transponder manufacturers don't hide the fact:
How to Safely Clean Aluminum Parts with Ultrasonic Cleaners
Low ultrasonic frequencies are commonly used to clean metal surfaces, but these will damage aluminum parts. Here are Kaijo’s recommendations for Aluminum.
Industrial cleaners usually have the ability to change both the frequency and the power of their transponders - a luxury we don't have in our toy ultrasonic cleaners.
Aluminum alloys however vary in composition and physical properties, and in general I'd expect that if it's been used in a can, that it's probably OK. I've used an ultrasound on my Beretta 92 frame many times, and the anodized alloy they use for that frame is fine. Your best answer however is to contact the can manufacturer if you're at all concerned.