Good topic, I've been thinking about this kind of thing a bit lately; I may be running a game soon and would like to aim for having at least one set piece encounter each session that emphasizes the dynamic battlefield element of 4E (you know, that element that WotC modules seem to ignore completely when they stick you in ten foot wide corridor after ten foot wide corridor - at least in the WotC modules I've played).
Here are a couple of ideas I've jotted down.
Geysers
A bog with some sort of random geysers, think the Fireswamp in Princess Bride. The DM marks a few squares with tokens each round, either randomly or by choice, and the combatants notice that the ground in those squares is bubbling or churning. The round after a square is marked, roll a d6: on 4+ the square erupts into a man-sized flame geyser that attacks anyone in that square against Reflex, doing some amount of fire damage if it hits (possibly with some ongoing) and half damage if it misses (no ongoing). If it doesn't erupt then next round it's 3+ to erupt, then 2+, and stays at 2+. I'd allow everyone to notice the bubbling without any checks as the idea is to encourage movement.
Morphic
(This one's a little crazy but could be a lot of fun, I think. There may be some needless complexity as I've just tidied up the notes I made. This was inspired by an idea for a Star Trek/Spelljammer style campaign structure, with the party crewing a ship that sails the Astral, so environments and situations would vary greatly from adventure to adventure as they do "away missions" on random bits of astral flotsam or venture into other planes.)
This would be a battle on some chaotically shifting battlefield, something like the plane of Limbo, perhaps. It begins on a seemingly stable island floating amidst the roiling chaos. The DM inserts the island at the top of the initiative order at the top of round two. Each round on the island's turn, the DM rolls a d6.
1-2: Movement
The whole island lurches in one direction, roll d10 to determine the direction of shift, with 1-8 being a cardinal direction, 9 being down and 10 being up.
If the result was 1-8, roll d3 and shift everyone (even those not on the island, as the island is moving towards or away from them) that many squares (minimum 1 regardless of forced movement reducing abilities). If this would push someone off the island, normal rules apply.
On a 9, the island "falls" d3 squares, on a 1 there is no damage but everyone needs to save or fall prone, on a 2-3 there is d10 damage and everyone falls prone (Acrobatics can reduce). Fliers who were standing on the island can choose to hover rather than fall if they're able to do so.
On a 10, the island moves up and the increase in G force causes all squares to be treated as difficult terrain for that round.
If there has been a split (see 5) then roll d6, on 1-4 the sub-islands can move as a group, on 5-6 they move individually (although moving sub-islands individually could be cumbersome to manage, so it might be easier just to move them as a group).
Only combatants are moved, effects/zones stay in place on the island.
3: Reduction
The island shrinks, losing one square on all edges. If someone is on a removed square, treat it as if they were pushed off the edge of an adjacent square, if there are no unoccupied adjacent squares available they're out of luck. If there has been a split, roll d6, on 1-4 one island shrinks (randomly pick one), on 5-6 they all shrink.
4: Expansion
The island grows, gaining one square on all edges. If there has been a split, roll d6, on a 1-4 one island grows (randomly pick one), on 5-6 they all grow. This could rejoin sub-islands.
5: Split
The island splits. DM determines where the island splits, either randomly or by choice. Move the edges of the split one square apart. If there has already been a split, randomly determine which sub-island will split this time.
6: Join
The island rejoins. If there has been more than one split, roll d6, on 1-4 randomly pick two adjacent sub-islands to rejoin, on 5-6 they all rejoin. If there are no current splits, treat this as a split instead.
Miscellaneous
Anyone who has fallen off the island is pulled d3 squares towards the nearest edge at the start of the island's turn (subjective gravity, or whatever you want to call it).
Assume the island has enough depth that people won't end up underneath it. If they end up below the level of the island surface they're pulled back towards the surface, if the island grows and they're adjacent they're pushed one square, if the island moves sideways into them it hits them for d10 damage and pushes them (treat as a fall, normal rules apply), but they're then able to climb up the side to the surface and get back on.
People off the island are free to use abilities, although they can't move normally (non-standard movement, such as flight or teleporting, works fine). They can make ranged attacks or melee attacks if they're in reach. Combatants not on the island who don't have some means of controlling their movement (eg/ flight) suffer a -2 penalty their own attack rolls and grant Combat Advantage to anyone attacking them.
Having a bunch of tiles of various sizes would be handy for the changing island sizes and splits. Or, after looking at Weem's post perhaps cardboard cut outs of various sizes could be used for non-standard shapes, possibly even chopping up islands as splits occur.