Great! But would this mean to have the whole blockchain on your phone? Which is a nogo clearly. I know a lightweight solution like Electrum ist not as secure as a full node but there's really no other way for mobile devices I guess.
As JustusRanvier alluded to, Bitmessage doesn't have a blockchain, as there's no need for a distributed consensus. Instead Bitmessage has a set of shared network objects (messages and public keys), with nodes in the network retaining the last 2.5 days worth of that data.
You're quite right that phones are not suitable for processing all the data that a full node deals with, which means that we have to come up with a way to make a 'lite' client for Bitmessage. Thankfully I think that we now have a good solution for this, namely prefix filtering of the kind used in Bitcoin stealth addresses (see
http://sourceforge.net/p/bitcoin/mailman/message/31813471). I met Peter Todd a few days ago and he suggested this, so all credit to him.
So, we now have a decent method of creating lite clients for Bitmessage - it will just take some time to get it implemented. It's also worth nothing that lite clients could help us to mitigate the damage of flooding attacks (such as the one going on now) by shifting most of the burden of network processing away from end-users, in favour of volunteers and other groups that can run high-capacity full nodes. That's my hope anyway.