Or, if you have the new iPad with dictation turned on (and are connected to the internet), you can press the microphone key (left of the space bar), wait for it to beep, say 'hash tag', then release.
One is about as fast as the other. Ok, not really, but the dictation is funner, at least the first couple of times.
There are a lot of other hard to find symbols that work this way, provided you know the proper name. I should make a list.
For symbols that I use often, like the underscore key, I just create a keyboard shortcut. For example, I get an underscore by typing two characters -- uu.