(Oops just noticed your question was for XLR, but I'll leave my response up for any RCA owners that want a cheap tweek?)
Prior to actually buying some...
I'm sure you have a box in your closet full of dozens of cheap generic RCA IC's that come with every electronic component which your likely never ever to use!
Take few sets and cut off the cable, hit the ends with a cigarette lighter to smolder them shut. Now plug all the unused in/outputs and see if you hear a deeper quieter "blacker" background?
I've tried RCA shorting plugs as well as non-shorting caps on the open inputs of one preamp I was having noise floor problems with. Note, you don't want to short open output connectors, only inputs. I noticed very little though some improvement. I made my shorting caps using old generic IC's as Audiobugged suggests (though I soldered the cable ends to ensure they indeed were shorted). This is an easy and free way to test for yourself.
I can't address your question related to balanced inputs.
Made my own shorting plugs to save money and I did notice an improvement despite still having open balanced inputs. You won't notice the difference right away, until you remove the plugs. Then, you'll put them back in immediately realizing that they did, indeed, lower the noise floor noticeably. Definitely not snake oil.
Chad: My gear is all un-balanced so I cannot address your XLR question. However, below is a quote from member Nsgarch, which may still be buried somewhere in the archives:
"It is customary to short any unused RCA inputs on preamps. This prevents pops and clicks when switching your source selector between actual sources that are plugged in. Also prevents unused inputs picking up stray RFI. You can use pre-made shorting plugs or make them yourself. I don't recommend the Cardas caps because they don't actually short the input, but only shield the hot signal socket from RFI.
Unused XLR inputs do not require this treatment since they are balanced and self-cancel any noise."
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