I use openid not to have a single signon to 37signals apps. I use to to have a single signon period. Not just 37signals but a ton of other apps use it as well (and I wish all of them did).

Every time I see a web app supporting openid Im glad that I don't have to invent yet another user/password combo. again.

As to failing openid providers I have a good suggestion - use OpenId delegation to have a single openid that you can reroute to any openid provider you want. all it takes is a domain name and a very small file hosted on S3 (for example). Then you can switch providers at will.