"Why did it end in my BULK folder if it really came from Yahoo?"
Because it almost certainly didn't. Can you PM me the headers ?
"what's a PIF file?"
Program Information File, very basically, used to tell Windows how to run another program. When you doulbe-click on a PIF file, it will run the program that it describes. But since the PIF doesn't contain the actual program (it kind-of points to the real code) it will
not show up as a virus itself, which fools some AV software and even more people, which is why the virus writers use it, of course...
"what happened to the button they used to have to report spam?"
No idea -- don't use yahoo
"Am I right to be suspicious?"
Very

But it's probably safe to just ignore / delete this one...
"Anyone else get this?"
Nope
"Who says "Cheers" in an important email?"
I might, but not in this case
Hope this helps a little
/RTFM
PS You need to spell it "[c o l o r =" not "[c o l o u r =" 'cos this software is American...