WEP [40 bit or 128 bit] is not secure at all; this is due to a flaw in the size and location of the keyspace used to encrypt the traffic with the RC4 algorithm.
See this research by a colleague of mine for more information. As far as default Linksys/D-Link router configurations, they are absolutely not secure by default. These SOHO vendors are notorious for compromising security for ease of use. Many security features ship 'OFF' by default, including WEP for wireless capable routers from companies like Linksys. They are also known to ship with default passwords, poor access control, etc. Browse archives.securityfocus.com 'pen-test' or 'bugtraq' lists for more information. Things you should do:
1) Upgrade your device to the latest firmware as soon as you get it out of the box.
2) Thoroughly read through and make sure you understand all the configuration options
3) change default passwords and secure the router as much as possible through its options
4) See if your router supports syslog logging, set up Kiwi syslogd on your Windows system and monitor your logs to see who's banging on your front door regularly.
5) Keep up to date on firmware revisions and security bulletins from your vendor.
Security is a journey, not a destination