I did a reboot after the flashing MIUI and only then flashed siyah. Should I try to apply both zips at the same time?Worked fine for me. Did you do a reboot after updating to 2.5.4 or just install the kernel straight after update?
[EDIT] Just tried flashing them both at the same time (process wipe, flash MIUI, flash siyah), and it's hanging at the MI logo. What order did you do it in?