Yes you will need it soon as SpinDry pointed out above that Punkbuster will be running on the server soon.
If you have the Punkbuster files/upgraded from version 1.0 you should be fine otherwise follow the guide for installing Punkbuster from above.
look in Call of duty 4\pb directory and you should see a listing similar to this if Punkbuster is installed
<DIR> dll
<DIR> htm
<DIR> scrnshot
<DIR> svlogs
<DIR> svss
65,536 pbag.dll
65,536 pbags.dll
6,036 pbcl.db
887,761 pbcl.dll
897,653 pbclold.dll
956,558 pbcls.dll
887,761 pbclsold.dll
120 pbns.dat
3,504 pbns_c.dat
40 pbsv.dat
2,434 pbsv.db
425,984 pbsv.dll
434,176 pbsvold.dll
103,736 PnkBstrB.exe
22,328 PnkBstrK.sys
530 sv_viol.log
""if not installed"" you will see this which is 100% of steam users
1,314,816 pbsetup.exe
674,600 pbsvc.exe
or the PB directory will not exist or will be empty in some cases.