why is my file system suddenly read only?
/system is almost always read-only by default. Just remount it as R/W:

# mount -t ext3 -o rw,remount /dev/block/mmcblk0p25 /system


There's also at least one app that will attempt to do this for you, but it may not work on all devices.

Another option is to use adb root (if your ROM will allow it) to restart adbd as root, then simply issue adb remount. On ROMs where adbd is already running as root, the first command is unnecessary and adb remount will work right off the bat. From a PC:

C:&gt; adb root
* Some response about success *
C:&gt; adb remount
Remount succeeded


As to why it's getting blown away, is your device NAND unlocked? Many HTC devices will attempt to restore their /system partition to a stock state if they are NAND locked, making changes to /system fleeting at best. That's something of an educated guess, but the fact that you noted that you are using VISIONary to temproot seems to support that hypothesis.