Forum Moderators: open
If it's a slideshow, you can provide alternate content simply by providing a listing of thumbnails with captions and links to the larger image. It won't be a "slideshow" in this alternate form but it will still let people with javascript disabled view the images. If you use proper alt="" tags on the images, then you still have content for folks who are visually impaired.
Now, to answer your actual question... ;) From a technical point of view, whatever message you want to include for folks without javascript can just be placed inside of a <noscript></noscript> block.
As far as being polite, I think that has to be up to you and how you want to talk to those folks. No matter what, you're delivering bad news (that they don't get to experience what everyone else experiences).
HTH,
Gregg