I would think that the act as a barrier rather than as a repellant, but am not 100% sure. I like them just because I like the feeling of having something on top of me, and many places in Asia don't provide top sheets. Also - they help keep mosquitoes away as well.
Some people soak them in pyrethin to help repel bugs - that may work for bedbugs? I think I soaked mine on my last trip, but won't be doing so on my upcoming trip. I don't like the idea of sleeping amongst the chemicals.
I agree with lizzy that it is more likely a barrier than anything else. I purchased a pair of silk liners from a company in New Zealand, where that trouble-maker lizzy lives, and they've been great. They only weigh a few hundred grams, if that, and are useful for an added bit of warmth on those chilly overnight sleeper train rides and for a barrier against things like bedbugs or, more frequently, questionable sheets. My wife loves hers, and I've never heard any negative comments about them. I ordered mine directly from New Zealand, and luckily it was actually delivered to me via US mail, so I guess I dodged a bullet there. Cheers.
"... where that trouble-maker lizzy lives...."
That's kinda funny exacto, because what you didn't know is that I was called Lizzy Trouble-maker at school because it rhymes with my last name!
I paid about 12 euros for a silk liner in a large sports shop chain in France and it was well worth the money. Came in a small drawstring bag so it takes up minimal space and weighs practically nothing. It's so much nicer than using a sarong (or two) as makeshift sheets, less bulky, dries quicker and doesn't get scrunched up or slip off during the night. It's an essential piece of kit as far as I'm concerned but it does depend on what kind of accommodation you'll be using of course.
Thanks for the feedback re soaking in repellent Lizzy. I was thinking of doing that next trip, but probably won't bother now!
Just for the record, bed bugs are becoming more and more resistant to pyrethroids, and the amount of repellancy they offer depends on the formulation. So, in other words, using them won't ensure that BBs stay away.
With liners you will have a physical barrier, but it's not 100% effective unless you can zip yourself up inside it. Bed bugs are attracted by heat and CO2 and if they're on the surface of your liner they'll probably find a way inside. Still, better than nothing, I suppose.