Glad to hear!
It basically lands on the to: bar whenever you make a new email. (So a brand new email, right-click in the from column, right-click a header address, or contact in address an book, etc, etc.)
Why is it done that way?
A) less 'situations' to code

B) So you can quickly add more recipients. Then you only need to hit tab twice to get to the subject line. (or 3 times if you have the bcc: field showing)
That's easier than shift-tab twice to get back up if you had instead started in the subject bar. Plus, you would be surprised at how many end-users don't know that shift-tab goes in the opposite direction of tab.
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Random notes/thoughts:
You'll also notice that more and more tab-landings are being slowly being added. Though personally, I'll just be happy when that QuickAdd Appointment dialog can tab or at least start in the subject area. Instead/as a temp workaround you can use 'n' for new, then simply hit tab once to get to the appointment subject line.
If your on an older version, you might turn off QuickAdd so it's doesn't pop-up on double-click.
And if your using a version that has 'more details' you could hit enter to open the full scheduling window, then tab once to start entering your appointment's subject line.
I can completely navigate with the KB or use the zimbra assistant (press `or ~) except for the calendar, as at some point you have to still resort to mouse. But this will get better-ie if the calendar area had focus so you could use arrow keys, but I believe all these RFE ideas are already in
bugzilla.