Having decided to register as a developer, I went to http://create.msdn.com and filled in the form. I was irritated to find that, the credit card I had stored against my Zune account having expired, no mechanism was provided for editing it, and that I therefore had to add a second credit card.
My account is now awaiting external validation via an organisation called Geotrust; despite my credit card having been debited, I am not allowed to unlock my 'phone until this validation is complete. This is annoying - surely Microsoft only needs to check I am a fit and proper person (which I assume to be the point of all this) when I attempt to submit an app? As it is, I am allowed (indeed encouraged) to submit an untested app, but not to test one. Ah well.
I am promised action within 2 working days, so will post again when I have unlocked my 'phone.
Meanwhile, I have downloaded both the WP Developer Tools and WPDM, and installed the Developer Tools.
The Developer Tools are 650MB in size. They took all morning to download and install, but my PC only has 2GB of RAM when the pre-reqs say 3, so this is probably my own fault. I haven't tried any of it yet (I am especially looking forward to using the WP7 Emulator and to seeing the new 'Mango update' functionality that will be generally available in the autumn). I haven't used a non-Eclipse IDE since the early nineties, so this could be interesting!
The pre-reqs for WPDM specify an unlocked 'phone, so I am leaving that install until Geotrust have done their bit. Interestingly the Developer Tools are also a pre-req, which doesn't augur well.