Update on my situation:
I have a T-Mobile HTC Desire that I unbranded using a generic HTC rom (through the process of making a "Goldcard", then downloading the RUU file and running that on the PC). That all went smoothly but when I then did this most recent update, I lost the WiFi.
It seems that this IS an issue with unbranded devices. Devices that come from HTC with no branding originally are fine.
I had to re-flash back down to the generic HTC firmware to get WiFi back, but am pleased to say that I have now managed to update to this newest firmware.
I used the Goldcard I made previously, and the firmware located here
RUU_Bravo_HTC_WWE_1.21.405.2_Radio_32.36.00.28U_4. 06.00.02_2_release_126984_signed.ex - xda-developers <- the first link is to the RUU file.
The process is - turn off phone, remove sim card and microSD card. Put in Goldcard (have a search around for the instructions for making a Goldcard, it's not hard). Turn on phone, connect via USB, run the RUU....exe file downloaded. That wipes the handset and installs this new firmware, re-boots, etc.
Turn off, put your correct microSD card in, turn on, set the phone up WITHOUT the sim card in (first time I left the sim card in and it put some Virgin mobile rubbish on the phone, even though it was a T-Mobile handset), then power down, put sim card in and away you go.
Am now on 1.21.405.2 with working WiFi and everything.