[DaphneDiane]

Okay I created a new non-admin test account (only preference change was keyboard to dvorak), set up OmniWeb (Set the "Start Page" setting, split out the location field, logged into these forums and saved my password, closed all windows and quit so I could start clean)

I got exactly the same results. I also had previous tried using the space bar, enter keys etc when the selection vanished and nothing happened.

For comprasion I tried the test under OmniWeb 5.1.3.

Major differences are as follows
Forward tab from location field on blank page, only cycles between selection vanishing and the location field, only way to get to search field is the shift tab. (My habit for searching under OmniWeb 5.1.3 was Commannd-L followed by shift-tab)

Once the page loaded, again the cursor vanished (sorry I didn't mean to put an asterisk on number 10). However the first tab went directly to the title field.

Summary:
Forward tab:
5.1.3
title -> message -> search -> location -> *vanish* -> title->...
5.5 sp 7
title -> message -> message -> notification -> search -> location -> title -> ...
Backward (shift) tab:
5.1.3
title -> location >search->*vanish*->message->title-> ...
5.5 sp 7
search -> notification -> message -> message -> title -> location*
(all further stay at location)

Also for 5.5 sp 7 I have occasionly had it where if you click on message the "no icon" button get into the loop as follows. (Unfortunately I don't have a repeatable set of steps to get this)
tab: message->no icon->notification then goes to the pattern mentioned above.
shift tab: message->no icon->message->no icon -> repeat.

I was runninng this on my intel iMac 20"