If nothing else, do a full recovery mode reinstall. Probably the cleanest way to do an OS upgrade anyway.
iOS4 wifi works fine on my 3GS.
Question...how many GB's is your 3GS? My daughter upgraded from 3G with 6GB free space to 3GS (iOS4) and now has 3GB free space with the same apps. Now her 3GS is locking up quite frequently. I tried to convince her to go with the 16GB and just wondered if she should trade it before her 30 days is up. Kids...