Er... the negative prefix in Spanish is "in-", so derivations such as "inmoral", "innegable", "intranquilo", "incorregible", "indecente", "inquieto", "ingobernable" are merely following the grammatical pattern. There is no dissimilation here.
A special case, as with many other languages, is that of the liquid consonants "r" and "l", which often modify preceding sounds. Because of the presence of these sounds we get words like "ilegal" and "irregular", where what happens is, in fact, assimilation of "in-", where the "n" disappears. In "irregular" the double consonant appears so that the lexeme "regular" can retain its initial-"r" sound, which is strongly rolled, as opposed to mid-word "r" which is softer. In the case of "ilegal", the "n" simply disappears.
And finally, the other case for exception to the rule is that of the combination "np" and "nb", which in Spanish is always "mp" and "mb" (the "n" sound becomes a bilabial in order to be closer to the "p" and "b"). It is a rule of spelling, so it doesn't only happen with the prefix "in-", but in every other instance too.
However, I'd like to propose the concept of "euphonic assimilation" for the inital question in this thread. I guess it's more or less what y'all were proposing, but I kinda like the name...
I really hope I explained myself adequately above (and I hope I'm saying something relevant); if not, do ask me to clarify and I'll do my best.
