I have anacdotal evidence that BT and Sunwell have both been 5-manned. As a result, the bosses that I've said are hard must be achieveable, but they are probably require high dps or cunning tactics.
Karazhan - Mostly trivial. Netherspite requires beam co-ordination which raises the number of people required.
Zul'Aman - Again trivial.
Gruul's Lair - Maulgar will be tricky, but doable. Gruul needs around 6667 raid dps to kill before 17 grows. Easy if you have 2 tanks.
Magtheridon - Trivial once you've cleared the channellers. The Blast Nova that needed interupting otherwise it wiped the raid only does 12.5k damage per person.
SSC - Vashj is possible, but you'll need to be very fast spotting tainted elementals before they despawn. Co-ordinating the Phase 2 should be the challenging bit. Everything else should be doable.
Tempest Keep - Al'ar, requires 2 tanks + 1 healer tank? Voidreaver is easy (needs 8k raid dps), Solarian shouldn't be a problem. KT would require some creative thinking, but might be possible with 5. He would be a challenge to work out tactics.
Mt Hyjal - Azgalor will probably be the blocker here. He blows someone up every 45s. Should be do-able up to him. At a guess Kaz'rogal will need 20k raid dps which might be a challenge, depending on if regen exceeds mana loss from marks every 10 seconds.
Black Temple - Naj'entus and Supremus should be ok. After that you can choose the kill order. Akama needs 17k raid dps. EoS needs 19k raid dps. Gurtogg Bloodboil's bloodboil is less of an issue so he is doable. Teron Gorefiend kills someone every 30s after 1.5 minutes, so he'll probably be the blocker, although he only has 4.1m health.
Sunwell - Don't go there.
Doomwalker can be soloed at 80 by some classes.
Kazzak - Shouldn't be a problem.
Hopefully someone can provide some feedback on those opinions.