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C# (101 characters)

string I(string i){int s=0,j=0,f;for(;j<12;)s+=(i[j]-48)*(++j%2>0?3:1);f=10-s%10;return i+(f>9?0:f);}


string I(string i)
{
int s = 0, j = 0, f;
for (; j < 12; )
s += (i[j] - 48) * (++j % 2 > 0 ? 3 : 1);
f = 10 - s % 10;
return i + (f > 9 ? 0 : f);
}


Tested on several ISBNs from books on my shelf, so I know it's working!