Is the "wa-particle" in katakana ワ or ハ？
(Just a quick question - I think I have seen both used, but if ワ is correct then I have just seen the results of a computer o/p automatically using ハ)
The basic answer is that は is written ハ in katakana.
However, I think it depends on why it's written in katakana. One reason you might write something in katakana is to communicate pronunciation, and in this case the particle は would be written ワ:
You can see this sort of use of katakana in some official documents, for example.
On the other hand, if you're just writing regular Japanese using katakana, you'd usually spell は as ハ. Another reason you might use katakana is for the use-mention distinction; some books or papers about Japanese grammar, for instance, write particles in katakana when they're mentioned rather than used, and in this context は would be ハ (and を would be ヲ, and so on).
Katakana is just used for foreign words, the grammar elements are not written in Katakana.