You can use offsets to make a column appear centered, just use an offset equal to half of the remaining size of the row, in your case I would suggest using col-lg-4 with col-lg-offset-4, that's (12-4)/2.
<div class="row">
<div class="col-lg-4 col-lg-offset-4">
<div class="input-group">
<input type="text" class="form-control" />
<span class="input-group-btn">
<button class="btn btn-default" type="button">Go!</button>
</span>
</div><!-- /input-group -->
</div><!-- /.col-lg-4 -->
</div><!-- /.row -->
Note that this technique only works for even column sizes (.col-X-2, .col-X-4, col-X-6, etc...), if you want to support any size you can use margin: 0 auto; but you need to remove the float from the element too, I recommend a custom CSS class like the following:
.col-centered{
margin: 0 auto;
float: none;
}