AFAIK WPA uses WEP for the encryption but it has much better authentication schemes and does automatic rekeying so a bruteforce attack will only be worthwhile if it can be done very, very quickly. But in general if you have WEP enabled it'll be 'good enough' as long as there is a non-protected WAP around, it's a lot less work to connect to another AP than it is to break into yours for no reason.