Rust on chain and cassette It looks like rust on my chain and cassette.

Both were new 5 months (4000 km) ago. The chain is about 60% worn/stretched. I've been riding 5 days/week, 40 km/day, Toronto on mild winter roads, and using only 'dry' lube (i.e. alcohol and teflon) on the chain.

What should I know about that?

I like to keep my bike in good working order, so that it rides very well (but, arriving home each evening after an hour-long commute, I have had no time for bike care: I just lock it, in the laundry room). Is this rust only cosmetic?

Should I use a winter lube? What (product or servicing) should affect/prevent rust on the cassette, if that's necessary?

When I replace the chain (when it's next serviced, perhaps two months from now) should I ask the LBS to replace the cassette again as well?
It's unclear how much rust you're talking about, but it's unlikely that a bike could develop serious rust on a chain in 5 months unless left outside with no maintenance.

I've dealt with some pretty rusty bikes over the years (especially when I spent a spell working on donated bikes for Christmas Anonymous) and I've never seen a cog that was so rusty that it was unserviceable.

The chain, of course, is a different matter, but generally rust on a chain is of little consequence unless it causes "stiff links", and that won't happen on a bike that's used regularly.

I'd recommend that you switch to a slightly less "dry" lube (or at least give the chain a few squirts of regular oil every now and again).  Also, if you've been cleaning the chain at all, be a little less diligent cleaning it -- it's not necessary to get it shiny bright, you just want to get off the crud.

Otherwise don't worry about it.