Does the sd-ext partition get wiped out after flashing a different ROM? I have a Samsung Galaxy Ace using its official CyanogenMod build. I wanted to try this MIUI ROM for a change: 

http://forums.miui.us/showthread.php?22816-ROM-MIUI-us-2-8-3-for-galaxy-ace

Everything went fine, except that it doesn't see the 1GB sd-ext partition now. I don't know if it's just unmounted because my sdcard partition size stayed the same. Does the process really do that?

I'll check the recovery menu once I'm done backing up my sd card. But if you have any ideas, please do share them. Thank you.

Edit: Nothing in the recovery menu helped. I'm now about to re-create the sd-ext partition.
I think your build f MIUI doesnt have the code for mounting the SD-ext partition on boot.
To check this, open the init.d folder in your new ROM and see if it has any file relating to Sd-ext.

This link has a init.d script for mounting the SD-ext partition link