Hosting provider SoftLayer was partially taken down this morning from a DDOS attack, and several well known websites, including TechMeme and TwitPic, went down with it.

The problem at SoftLayer was resolved, but some users of OpenDNS, a DNS service provider that is becoming more and more popular, still can’t reach those websites. The reason? OpenDNS caches IP addresses for domain names on a user’s computer, and they’ve cached a bunch of bad DNS entries now on these computers. This speeds up web surfing considerably, and has helped some users avoid major outages at the ISP level in the past. But in cases like today, with outages at the hosting level, the bad IP information ends up being cached for up to a day.

Users who know what’s going on can reboot their computers to clear the cache, but that’s clearly not a good overall solution. OpenDNS says they are turning on a feature called SmartCache that caches both the current and “last good” IP address, so situations like today won’t be an issue any longer.