Pass asp.net ClientId to Javascript Function - inline as a param
<asp:ImageButton ID="ImageButton3" runat="server" ImageUrl="images/butt_searchoff.png"
    class="sb_search" ToolTip="Search the Database" AlternateText="Search" OnClick="ImageButton3_Click" OnClientClick="SetExpandedCount()"/>

    <input id="tbSearch" runat="server" class="sb_input" type="text" autocomplete="off" onkeypress="return OverridePostBackOnEnter(event, '" & ImageButton3.ClientID & "');" />

function OverridePostBackOnEnter(event, ctrl) {
        if (event.keyCode == 13) {
            alert(ctrl);
            if ($.browser.mozilla) {
                __doPostBack(ctrl, 'OnClick'); //for IE
            }
            else {
                //but for other browsers you should use
                __doPostBack(ctrl, 'OnClick');
            }
        }
    };

<asp:ImageButton ID="ImageButton3" ClientIDMode="Static" runat="server" ImageUrl="images/butt_searchoff.png" class="sb_search" ToolTip="Search the Database" AlternateText="Search" OnClick="ImageButton3_Click" OnClientClick="SetExpandedCount()"/>

<input id="tbSearch" runat="server" class="sb_input" type="text" autocomplete="off" onkeypress="return OverridePostBackOnEnter(event, 'ImageButton3');" />

function OverridePostBackOnEnter(event, ctrl) {
        if (event.keyCode == 13) {
            if ($.browser.mozilla) {
                var overridctrl = document.getElementById(ctrl);
                __doPostBack(overridctrl.name, ''); //for IE
            }
            else {
                //but for other browsers you should use
                var overridctrl = document.getElementById(ctrl);
                __doPostBack(overridectrl.name, '');
            }
        }
    };

<input id="tbSearch" runat="server"
                  class="sb_input" type="text"
                  autocomplete="off"
                  onkeypress="return OverridePostBackOnEnter(event, this.id);" />

<input id="tbSearch" runat="server"
   class="sb_input" type="text"
   autocomplete="off"
 onkeypress="return OverridePostBackOnEnter(event, '<%#ImageButton3.ClientID%>');" />

OverridePostBackOnEnter(event, this.id);

OverridePostBackOnEnter(event, this);

OverridePostBackOnEnter(event, '<%= ImageButton3.ClientID %>');

<asp:ImageButton ID="ImageButton3" runat="server" ImageUrl="images/butt_searchoff.png"
    class="sb_search" ToolTip="Search the Database" AlternateText="Search" OnClick="ImageButton3_Click" OnClientClick="SetExpandedCount()"/>

    <input id="tbSearch" runat="server" class="sb_input" type="text" autocomplete="off" onkeypress="return OverridePostBackOnEnter(event, '<asp:Literal ID="otherControlsClientIDHolder" runat="server" />');" />

otherControlsClientIDHolder.Text = ImageButton3.ClientID;