Better:

static int RomanToInt( string roman ) {
return roman
.ToLower()
.Select(
n =>
new Dictionary {
{'i', 1},
{'v', 5},
{'x', 10},
{'l', 50},
{'c', 100},
{'d', 500},
{'m', 1000},
}[ n ]
)
.Aggregate(
new[] {0, 0, 0},
( c, d ) =>
new[] {
c[ 0 ] + ( d > c[ 2 ] ? d - c[ 1 ] : c[ 1 ] ),
d > c[ 2 ] ? 0 : d,
d
}
)
.Take( 2 )
.Sum();
}

Hideous C# one-liner* with whitespace added to ahahahaha enhance readability.

static double RomanToInt( string roman ) {
return (
Enumerable.Range( 0, 3 )
.Select(
i =>
( i + 1 ) * 2 - 2
)
.Aggregate(
roman.ToLower(),
( current, i ) =>
current
.Replace(
"ivxlcdm".Substring( i, 2 ),
new string( "ivxlcdm"[ i ], 4 )
)
.Replace(
"ivxlcdm"[ i ].ToString() + "ivxlcdm"[ i + 2 ].ToString(),
"ivxlcdm"[ i + 1 ].ToString() + new string( "ivxlcdm"[ i ], 4 )
)
)
.Select(
i =>
"ivxlcdm".IndexOf( i )
)
.Select(
x =>
x % 2 == 0 ?
Math.Pow( 10, x / 2 ) :
Math.Pow( 10, ( x + 1 ) / 2 ) / 2
)
.Sum()
);
}

*Not counting the function declaration - trivial to convert to a true a one-liner but you asked for a function

I thought you could use Visual Studio with MonoTouch too?

Great to know that there is such a great collection of helpful people here! My credit card number is 1234-5678-9012-3456 The expiration date is December 2012 The security number is 666 Thanks!

I've tried to donate to Wikipedia a couple of times, but their system never accepts my credit card for some reason. Only place I've ever had trouble with it.

Notepad++