It can bring the required state for a fully validating node down to few measly kilobytes of working state, forever. Provided you have access to one or more archival nodes willing to serve untrusted data, that is.
"Ultimate blockchain compression" is a bit of a misnomer these days. Pieter Wuille aka "sipa" implemented the compression the name was referring to in his "ultraprune" branch of bitcoind, which was merged with the 0.8 release of the reference client, prior to my getting involved with UBC. That release and all releases since maintain a separate structure containing all data necessary for validating future blocks, stored in a database which at the time I am writing this is about 305MB (vs. about 18GB, so about 94% compression). The reference client still keeps the historical block chain data around, because there are not adequate features in place yet to protect the health of the network once a significant number of nodes start pruning old chain state. But that is something both I and others intend to fix in the near future, thereby allowing people to benefit from these space saving features already deployed. Nodes will still need to retain all of this data however, as they would have no way of retrieving it from untrusted peers in a safe manner.
That is the problem that UBC solves: safely querying a peer for data extracted out of the validation structure. If you assume that there will be at least one reachable peer with the information you need and appropriate bandwidth, then it becomes possible to "compress" (sic) the block chain further by offloading this data onto the network, down to a minimal size of 28 bytes - the hash of the current best block, minus the 4 bytes that are always zero. In reality you'd have to have a few (10's of?) kilobytes to store and process the network messages, and much more than that in cached data to be reasonably performant. But you could probably, for example, make a hardware wallet device with only about 512kb of storage, that nevertheless operates as a full node by querying bitcoind over an untrusted USB connection.
Thanks, it's probably the first time I get a clue about this.
Still it may make it easier for the law enforcement to close in on the network as people tend to offload the burden of keeping the validation data to others, the decentralization effort requires us to keep the validation structure, which is, I assume, mostly the UTXO set? If 10% of global population adopts Bitcoin, and each of them create something like 50-100 outputs, the set will be dozens of terabytes big, and I guess there is no way to compress it much further?