Just my thoughts but if you're in top gear and at the kind of revs where you're wanting to go up another notch, in terms of safety it would surely be better just to drop the revs and try snicking up a gear than to take your eyes off the road look at gear indicator then look back up and re-focus on what's happening on the road? Yeah I appreciate that this scenario can happen at 40mph due to the great nature of the bikes low down pulling power but that's still fast enough to get you into trouble if you're concentration is not on the road.
But please don't let me be a killjoy if you fancy a new toy for the bike then by all means, but a gear indicator has never appealed to me mostly because of the above.
/Edit: If I was going to fit something of this ilk it would be Christian's multiguage which also happens to include not only a gear indicator but a lot more useful stuff - http://cvieth.bplaced.net/ganganzeige_summary.html
But please don't let me be a killjoy if you fancy a new toy for the bike then by all means, but a gear indicator has never appealed to me mostly because of the above.
/Edit: If I was going to fit something of this ilk it would be Christian's multiguage which also happens to include not only a gear indicator but a lot more useful stuff - http://cvieth.bplaced.net/ganganzeige_summary.html