If you want to avoid having $IFS involved, use $@ (or don't enclose $* in quotes)
$ cat atsplat
IFS="_"
echo " at: $@"
echo " splat: $*"
echo "noquote: "$*
$ ./atsplat this is a test
at: this is a test
splat: this_is_a_test
noquote: this is a test
The IFS behavior follows variable assignments, too.
$ cat atsplat2
IFS="_"
atvar=$@
splatvar=$*
echo " at: $atvar"
echo " splat: $splatvar"
echo "noquote: "$splatvar
$ ./atsplat2 this is a test
at: this is a test
splat: this_is_a_test
noquote: this is a test
Note that if the assignment to $IFS were made after the assignment of $splatvar, then all the outputs would be the same ($IFS would have no effect in the "atsplat2" example).