Thousands separator (comma) option for NumberString/StringCases?

I would just go to RegExp:

StringCases[" 1,142.123 ", RegularExpression["\\s[0-9,.]+\\s"], 1]

(* {" 1,142.123 "} *)

You could also go with:

ImportString[" 1,142.123 ","List"]

Though that will automatically change it to Numeric, which doesn't seem to be what you want...

This is admittedly a bit of a hack, but you could remove the commas first, using StringReplace:

StringCases[
 StringReplace[" 1,142.123 ", "," -> ""], 
 Whitespace ~~ NumberString ~~ Whitespace, 1]

(* ==> {" 1142.123 "} *)

Regular expressions are nice, but there is no need to use them here.

numString = (NumberString | ",") .. ;

StringCases[" 1,142.123 ", Whitespace ~~ numString ~~ Whitespace, 1]

{" 1,142.123 "}

---

The form above is admittedly not robust as a valid number string should not start or end with a comma or have two decimal points, and I believe the comma should not immediately precede a decimal point or appear to its right side. The easiest way I can think to preclude these is to check for them explicitly:

numString = x : (NumberString | ",") .. /;
   ! StringMatchQ[x, ",*" | "*," | "*.*,*" | "*,.*" | "*.*.*"];

StringCases[" 1,2. ,3 4, 5.6,7,.89 0.1.2sam, i am ", numString]

{"1,2.", "3", "4", "5.6", "7", ".89", "0.1", ".2"}