F#, 458 chars
Reasonably short, and still mostly readable:
let s=Array.init 10(fun _->new System.Text.StringBuilder())
System.Console.ReadLine().Split([|' '|])
|>Array.iter(fun n->
for i in 0..9 do s.[i].Append(if i%2=1 then"----"else" ")
let l=s.[0].Length
let i=68-int n.[0]+if n.[0]>'D'then 7 else 0
s.[i+3].[l-3]<-'O'
if n.Length>1 then
for j in i..i+2 do s.[j].[l-2]<-'|'
for j in i..i-1+(match n.[2]with|'4'->0|'8'->1|'1'->2|_->3)do s.[j].[l-1]<-'\\')
for x in s do printfn"%s"(x.ToString())
With brief commentary:
// create 10 stringbuilders that represent each line of output
let s=Array.init 10(fun _->new System.Text.StringBuilder())
System.Console.ReadLine().Split([|' '|])
// for each note on the input line
|>Array.iter(fun n->
// write the staff
for i in 0..9 do s.[i].Append(if i%2=1 then"----"else" ")
// write note (math so that 'i+3' is which stringbuilder should hold the 'O')
let l=s.[0].Length
let i=68-int n.[0]+if n.[0]>'D'then 7 else 0
s.[i+3].[l-3]<-'O'
// if partial note
if n.Length>1 then
// write the bar
for j in i..i+2 do s.[j].[l-2]<-'|'
// write the tails if necessary
for j in i..i-1+(match n.[2]with|'4'->0|'8'->1|'1'->2|_->3)do s.[j].[l-1]<-'\\')
// print output
for x in s do printfn"%s"(x.ToString())
