Injecting a sequence of expressions into a held expression

{3, 4} /. {x__} :> Hold[{1, 2, x}]

Hold[{1, 2, 3, 4}]

Leonid Shifrin used this here long before I wrote this answer.

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In light of Leonid's comment to halirutan it is worth pointing out that you can inject expressions from an arbitrary head including Hold. You can also use -> rather than :> like this:

expr = Hold[{1, 2, x}];

Hold[6/2, 2 + 2] /. _[x__] -> expr 

Hold[{1, 2, 6/2, 2 + 2}]

How about this:

ClearAll[inject];
SetAttributes[inject, HoldRest];
inject[Hold[{args__}], new__] := Hold[{args, new}]

This will also accept Sequence[3,4] as a second argument. Sequences are spliced, while arguments themselves not evaluated.

EDIT

You can also use a composite rule, with some head s instead of Sequence (you can localize s if needed):

Hold[{1, 2, x}] /. x -> s[3, 4] /. 
  f_[left___, s[middle___], right___] :> f[left, middle, right]

One way is to use Function and the possibility of SlotSequence. I define an additional function f to be sure nothing gets evaluated:

f[x_] := Print["Evaluated"];
Function[Hold[Do[f[1], ##]]][{i, 5}, {j, 5}]

(*
  Hold[Do[f[1], {i, 5}, {j, 5}]]
*)