Normal updates (e.g. timezone rule changes, updated maps, security fixes) should be covered under warranty service, and should (imnsho) always be at the owner's discretion (even if it's a simple checkbox "yes, install updates without asking"). Additional features could be treated as an up-selling opportunity for existing customers, or could be provided for free to create brand loyalty. A critical bugfix should be treated as a product recall/replacement and communicated as such, not pushed silently: I really do not want the software attitude of "let's ship it broken, we can always patch it later if customers complain" to trickle over into other product areas.

(Yes, I'm aware that part of the above means that software engineering needs to grow up and become a real engineering discipline. I'm OK with that, in fact I think it's overdue.)

Warranties on physical goods have a long history (in the US) of being void if the owner makes certain changes. Should that apply to software?

I'm struggling to think of an example where this doesn't apply already. Can you give an example where a software company has honoured a warranty request even with custom modification? Or even where a warranty request for a pure software product was honoured? Even Firefox, a good example of a software product that thrives on third-party modifications, requests that you try to reproduce bugs in safe mode, with all add-ons disabled, and they give no guarantees about following up on your report.

The only example I can think of right now is Microsoft's support strategy: they will bill you for the support request unless you can successfully convince them that it's a genuine bug in their software. But I don't think MS support tickets are an example of product warranty, since it's provided as an additional service anyway.