Hi Hardcore-fs,
Thanks for your observation on private key export and import from bitcoin-qt to MultiBit.
I have posted on your github issue raised but have included my reply here:
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Which version of MultiBit are you using ?
bitcoin-qt 0.7.2 exports compressed private keys (these begin with an L or K).
Before MultiBit 0.4.21 only the import of
uncompressed private keys was supported (these begin with a 5). If you tried importing compressed private keys with a version of MultiBit earlier than 0.4.21 you would see the behaviour you reported.
If you are on an earlier version try updating your copy of MultiBit and importing your private keys again.
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Generally I don't advise people to move private keys between wallets - the MultiBit functionality to export private keys and reimport them is there mainly to enable people to back up their private keys. Normally it is simpler to send the money (using a regular transaction) from one wallet to the other.
There are various things you have to be careful with (double spends, ensuring you replay the blocks from before the earliest ever transaction). Clients that have the full block chain (Bitcoin-QT, Armory, Blockchain.info and the Electrum servers) are better at private key work than MultiBit as they have the whole blockchain to look through. MultiBit only keeps data relevant to your wallet to keep the size of data down and speed things up.
To get back to your specific point though: Yes, this was a limitation with earlier versions of MultiBit. Let me know if you still have the problem using MultiBit 0.4.21 which does support compressed private keys.
Edit: It is a bit confusing there being compressed and uncompressed private keys. If you have a look at the
https://bitaddress.org and click on the 'Wallet Details' tab you can find out quite a lot about the different formats. Note: For maximum security you should save the bitaddress.org page to your local disk and use your local copy.
