It's even simpler than that. Ctrl-A (select all), Ctrl-K (Path->Combine).
The catch is that if you're using objects, they will be converted to paths (curves) and you'll lose useful things like fill, rounded corners, gradients... If you think about it, what you're asking only makes sense for very simple drawings - only those can be represented by a single path.
Ah, thanks for confirming, pretty certain that is what I ended up doing. If I recall, this did still have other issues, for instance handling scaling and origin was difficult, and also I think all the points had 10 decimal places unnecessarily. I needed to use web tools to get it into the right shape, although part of this may be that I was getting my head around the difficulties of how SVG works on the web, using viewBox rather than height/width attributes and so on.
The catch is that if you're using objects, they will be converted to paths (curves) and you'll lose useful things like fill, rounded corners, gradients... If you think about it, what you're asking only makes sense for very simple drawings - only those can be represented by a single path.