I thought I'd share my experience of this in case anybody else found themselves in a similar situation whilst doing Over The Air ROM updates.
So I got home from the pub last night and decided it was the right time to update my phone to Android 2.2 (hey, I'd had five pints, anything and everything sounded like a "good idea"!).
For background you should know that I spent last weekend making a gold card and flashing my Orange branded Desire to the v1.15xxx generic ROM to get rid of all the Orange crap. I did not root or unlock.
So... I checked for updates, and instead of being offered 2.2 Froyo, I was offered 1.21xxx. Curses! I know from reading on here that v1.21 breaks the wifi.
However, reading through the official Froyo thread in this forum I found a post by somebody in a similar situation suggesting that by upgrading to 1.21 and then checking for updates he was able to install 2.2. That was good enough for me (yes, five pints) so, with my gold card inserted, I accepted the update to 1.21 and left it to do its thing. Eventually it finished, but when it booted for the last time it said the SIM was network locked and I had to contact my operator.
OK, I thought, let's just fork out for a SIM unlock code, get this fixed and go to bed (it was now 2am). It didn't look like I had much choice anyway. However, before I clicked the final order confirmation button in Paypal I decided to try one more thing. I took my SIM out, booted the handset and then checked for updates. Fortunately my wifi still seemed to work because it offered me the 90Mb Android 2.2 download, which I accepted and installed. When it finished I took my goldcard out, put my normal SD card and my SIM back in, booted it up, and ta-daaaa! A nice fresh Froyo working installation and no network lock on my SIM any more. I don't know why it worked, but it did and it saved me twenty quid.
So the moral of the story? Drink more!