fwiw, you ought to be able to use any ant+ speed/cadence/hr sensors (eg Bontrager etc – a lot of them look like they're the same thing, rebadged – a speed or cadence sensor is about £20 each)
http://www.thisisant.com/
compatibility chart:
http://www.thisisant.com/modules/mod_product-directory.php
From what I read on a road forum in the last few days, if you've got the speed sensor fitted then it may well default to that, if it loses lock – not sure whether that's applicable to the 705, 500, or both..I may have the link squirreled away…
(I'd been looking at a Bontrager Node 1 or 2 for a cyclocomputer – on a recumbent, so wireless range/tidy wiring's an issue, hence ant+ gear – so now I'm wondering how useful the GPS mapping of a 705 might be. Tho' I've seen one thread rumour that a revised version might arrive at the end of the year. Hmmm…)
There's a decent review of the 500 here – additional q+a at the bottom:
http://www.dcrainmaker.com/2009/11/garmin-edge-500-in-depth-review.html
Supposedly the Oregon 400t and 300 support speed/cadence sensors, but curiously Garmin don't list them in their cycling products – I've yet to check what the differences are between them and the 500/705.