I have many frustrations with Google's multiple accounts. I use multiple personal accounts, a personal Google Apps, a business Google Apps, and a self-hosted (university) Google Apps.

I've found the best solution is to create bookmarks for GMail and Calendar with the URL like you mentioned, except you can use your email address in place of the number (that is, in stead of/b/3/ you can do /b/[email protected]/).

Not perfect, but I hope this helps!