Since you are new to this, I'll explain what is happening.
Google is sending out the OTA in waves. As I said above, I have 4 tablets in my house and only 2 have gotten the notification so far. But....Since you are on a Custom ROM, the OTA will inherently fail. There are a few reasons for this which I will explain below. Being that CleanROM is based off stock and that Scott didn't delete the OTACerts.zip mentioned above, Google's servers think that this is a stock setup and is sending the OTA to you. They have no idea that you have modified anything at this point.
Here is why it is failing and when Google realizes that the device is modded:
The OTA comes in and you download it. Once you flash it, the package looks to make sure that the device's integrity has not been compromised. If you have modified the /system/build.prop, it will fail. If you have flashed a different kernel or a recovery, the package checks for this and if it comes back as altered, the flash will fail. Sometimes it even looks at the apps in /system/app and /system/priv-app and if those are altered (including deleting and inserting of apks), then the flash will fail.
Long story short, unless the device is in virginal state, the OTA flash will fail. If you want to take the OTA, you're going to have to flash the factory.imgs through fastboot and then take the OTA that way. But realistically if you are going to fastboot a factory.img, you might as well flash the newest one and skip the OTA altogether (flashing imgs is always safer than taking OTAs). I just simply long-pressed the notification and blocked it, didn't download the OTA and will patiently wait for Scott to put out a new CleanROM when he is ready. You can delete the downloaded OTA if you decide either of the two options above, but I can't remember right now where it downloads to (maybe /root/cache I think).
Hope this helps and welcome to Android.