How does the lock screen work on the Desire? (beginner question)

You can set it all up to your liking.  15 seconds, 30 seconds, a minute, all the way up to ... well ... never

Also, there's a pattern lock (where you need to connect dots in a specific pattern you chose), a pin-code lock and a password to choose from.  So plenty of options.

It won't lock the screen while dialling, but does have a proximity sensor which protects you from accidentally clicking something with your ear while you're holding the phone to your head.

ohh, I forgot something.  What I said in my previous post is indeed the phone lock.  The security-related one.

There's also a secondary lock for the display, which is basically a slider you need to drag down.  This is also customizeable in time, with almost the same options.  15 seconds, 30 seconds, 1 minute, 2 minutes, 10 minutes and 30 minutes. 

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So for example : you set the phone lock for 10 minutes and the screen lock to 2 minutes.

2 minutes after you stop using the phone, the screen goes out.  To reactivate the phone you only need to touch the power button, at which point it will show the slider.  Slide it down and voila, you can do whatever you want.

If you don't use the phone for more than 10 minutes, it locks up completely.  First you push the power button, at which point it will show the slider.  Slide that down and it shows the keyboard, keypad or the dots you use for the pattern lock, depending on which security system you selected.  Only after entering the correct code/pattern, you'll be able to use the phone again. 

Some people think it's annoying that you need to double-unlock the phone, but after a week or so it becomes second nature.  You don't even think about it anymore (which is why I forgot to mention the regular screen lock in the first place.  It's THAT natural really).

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On the code/pattern input screen, there's a button for calling emergency services, so if you're involved in an accident, anyone can still call an ambulance with your phone without requiring your code/pattern. 

As for the manual, I assume that's the .pdf from HTC's site.  That's the same one we have, as the phone itself only comes with a quickstart guide really.  And the same .pdf you already read is on the memory card, so you can read it on the phone as well.  I'd suggest reading it on the PC tho, so you can use the phone to check things while reading. 
IMO it covers most of the functions, but it's too much info at once, and in the wrong order.  If you get the phone, play with it and learn as you go along.

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Thanks mucho for the good info and update. If it can do what you say re: display lock, I'll be very keen on getting an HTC Desire in place of the Cookie +.  

210 pages just about covers the basics.  A full rundown on every function would require an encyclopedia rather than a .pdf file.