Quick multiple selections from a list

This seems to give a rather decent performance (final version with improvements by jVincent):

Clear[getSubset];
getSubset[input_List,sub_List]:=
Module[{inSubQ,sowMatches},
    Scan[(inSubQ[#] := True)&,sub];
    sowMatches[x_/;inSubQ@First@x] := Sow[x,First@x];
    Apply[Sequence, Last@Reap[Scan[sowMatches, input], sub], {2}]
];

Benchmarks:

n = 10000;
biggerlist =  Map[{#, FromCharacterCode[Mod[# - 1, 26] + 97], #*100} &, Range[n]];
unsortedbiglist = RandomSample[biggerlist, n];
unsortedsubset = RandomSample[Range[n], Round[n/10]];

Row[{First[Timing[selection1=Map[Cases[unsortedbiglist,{#,__}]&,unsortedsubset];]]," seconds"}]

(* 1.170008 seconds  *)

(sel1 = getSubset[unsortedbiglist,unsortedsubset])//Short//Timing

(* {0.031,{{{8286,r,828600}},<<998>>,{{6420,x,642000}}}}  *)

selection1===sel1

(* True *)

 Timing[selection3 = Pick[unsortedbiglist, unsortedbiglist[[All, 1]], 
   Alternatives @@ unsortedsubset];]
 (* {0.218401, Null}  -- same as Cases[..., Alternatives@@ ..] *)
 selection3 == selection2
 (* True *)

Here is my take on Leonid's method. It's better because it's shorter and uses ~infix~. ;-)
(It's just a little bit faster, too: about 20% on his test.)

getSubset2[input_List, sub_List] := Module[{test},
  (test@# = True) & ~Scan~ sub;
  Apply[Sequence,
   Reap[Cases[input, x:{y_?test, ___} :> x ~Sow~ y], sub][[2]],
   {2}
  ]
]

getSubset2[Range@20 ~Partition~ 4, Prime ~Array~ 7]

{{}, {}, {{5, 6, 7, 8}}, {}, {}, {{13, 14, 15, 16}}, {{17, 18, 19, 20}}}

Although slower I cannot pass by the more direct implementation without comment:

getSubset3[input_List, sub_List] :=
  Last @ Reap[# ~Sow~ #[[1]] & ~Scan~ input, sub, Sequence @@ #2 &]

Also slower than getSubset2 but pleasingly clean, Association can be nicely applied to this problem in the form of GroupBy and Lookup.

getSubset4[set_, sub_] := Lookup[set ~GroupBy~ First, sub, {}]

getSubset4[Range@20 ~Partition~ 4, Prime ~Array~ 7]

{{}, {}, {{5, 6, 7, 8}}, {}, {}, {{13, 14, 15, 16}}, {{17, 18, 19, 20}}}