En réponse à william drewery :

> >
> > It doesn't really have /f/, tho.  [P] is an
> > allophone of /h/
> >
>True. But I can't think of any voiced equivalent to
>it. I'm guessing it's that restriction again.

[B] can be found only in borrowings that come with [v] in it, and then not 
for all speakers (many speakers borrow [v] as [b], Like Spanish :) ). 
Still, [B] appears often enough to have a way to write it down (a strange 
one at that. To write down a syllable beginning with [B], use the katakana 
for u /M/, add to it the voicing sign, and follow it with a small form of 
the katakana for the vowel of the syllable, except when this vowel is /M/ 
itself :) ).
En réponse à Danny Wier :

> > /w/ only occurs before /a/ (ban on /wi/ and /we/ is moderately
> >     unusual, ban on /wo/ and /wu/ is pretty typical)
>The Hiragana and Katakana syllabries actually do have characters for /we/,
>/wi/, and /wo/ (but still no /je/).

|we| and |wi| are archaic and unused in modern writing (and kept only for 
backwards compatibility :)) ). |wo| is only used to mark the object 
postposition which is pronounced /o/ (|we| and |wi|, when read out of older 
writings, are also pronounced respectively /e/ and /i/).

>  I think they're only used for foreign
>words (that aren't Sino-Japanese).

No they're not. Foreign [we] and [wi] sequences are usually borrowed as 
[M.e] and [M.i], i.e. they become bisyllabic (I don't have any example 
right now, but I do remember seeing such borrowings). If you find it 
strange, it's actually not uncommon for Japanese to borrow single syllables 
as bisyllables. All syllables with a consonantic non-nasal coda are 
borrowed that way ("up" is borrowed as |appu| for instance).

The ban on [we] and [wi] is very alive in Japanese (at least standardly :) ).

Christophe Grandsire.

You need a straight mind to invent a twisted conlang.