I've got a nice tub of copper-slip for putting bottom-brackets in, but there must be some other use for it… Any good on ally seatposts going into ally frames?

To be honest, unless you leave your seatpost and never move it for years (Say on a road bike) I'd say it's best left clean and dry. If it's not meant to move, why lubricate it? 🙂

I generally leave mine bone dry, especially on alloy frames.

but there must be some other use for it… Any good on ally seatposts going into ally frames?

pretty much every other nut/bolt/thread that doesn't need threadlock

I generally leave mine bone dry, especially on alloy frames.

Worried about it seizing- maybe it won't, being similar metals (no galvanic corrosion/whatever) but rather not take the risk!

The most I ever do with an ali seatpost in a ali frame is a quick wipe with GT85 soaked rag
More to make sure it's clean & grit free than lube it – but I guess it lubes it a bit

It won't sieze. Not if you move it more than once a year anyway. 🙂

It wont hurt on the seatpost, and even if it sounds contradictory it wont make the post slip in normal use or anything either. Get a bit under the seatcollar and the bolt and if its a qr, lube the mechanism on that with it too and they clamp up tighter for a given torque than they would bone dry.

Oh just remembered another good one is a smear of it on the outside of new bearings, they still fit nice and snug, but will be tons easier to remove when its time to change them again.

An old mechanic (like an old wife?) told me that copperslip is mildly abrasive and is okay on car wheel nut threads, back of brake pads etc but shouldn't be used in high tolerance locations.
Tiny bit of quality cycle grease will stop a seatpost seizing (NEVER on carbon post though!) but like other posters, never had one seize before even with alloy/titanium in wet+muddy conditons.

…never had one seize before even with alloy/titanium in wet+muddy conditons.

I've had an ally post seized in a steel frame- no way that one was coming out. The new build is my first experience of new-fangled frame materials (not steel, basically!).

I grease my ally seatpost in my steel frame. It got a bit stuck once and I don't fancy it getting a lot stuck. Normal Silko RG2

Plus I figure a bit of grease around the seatpost might limit the amount of water capilliary actioning down into the frame.

I've had an ally post seized in an ally frame, but it was left very mucky and wet repeatedly. I still never grease the post though, as it ALWAYS slips after that.

What happens when your ally post gets stuck in your similar metal frame…

I would always add copperslip (other anti-seize compounds are available) for this application. Also as said, any fixing that doesn't require thread lock. Pedal threads, cleat threads etc.

I never grease anything other than BB shells. Never needed to and torque figures are given for dry conditions unless otherwise stated if you use a wrench. Only thing thats ever seized on me other than the post above was cleat bolts, and they were drowned in copper slip.

I wouldnt use copper slip personally. Mixing ally with another metal tends to cause oxidisation. Just happened to a guy in our club when he used copper slip on his seatpost. Have a look here http://www.lanarkshiremtbclub.co.uk/index.php?option=com_fireboard&Itemid=56&func=view&catid=9&id=6606
Use a non metal based grease (I use TF2)and you should have ne problems.

I've had an ally post seized in an ally frame, but it was left very mucky and wet repeatedly. I still never grease the post though, as it ALWAYS slips after that.

What happens when your ally post gets stuck in your similar metal frame…

Pray tell, how long did you leave it untouched for it to sieze? (Genuine interest!)