If I'm already above the clouds -- how much higher can I go?

As of 1.2.1, the build limit doubled from 128 to 256, while cloud height appears to not have moved. This means you actually have more room to build above the clouds (about 128) than you do between sea level and the clouds (about 64).

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As of Beta 1.8, clouds now appear above the build limit. Notch also claims to have made the level height more consistent throughout the code, which leads me to assume that the clouds will always remain above the build limit regardless of that actual number.

I tested this by building the tallest possible pillar in creative mode. Standing on top of it (picture 1) puts your head just above the clouds. Flying beside it (picture 2) confirms that the clouds are above the build limit. Standing on a 1 meter shorter pillar puts the clouds just above your head, meaning that the clouds are about 1.5 meters above the build limit.

The clouds are at height 49-46 above sea level. That gives you a 15 meters' room above the clouds.

You can put blocks 64 metres (blocks) above sea level. Source. I don't know how high clouds are though