Why not just have an amount of BTC that Coinbase segregates as a 'security deposit'. Then that becomes the limit. So if you want a 250 BTC limit then you put 250 BTC into the security deposit. Then Coinbase would not have any risk of loss due to fraud because they would be holding the security deposit. This would be an easy way for them to get rid of the 30 day waiting period and find the honest customers easier.
This is actually a pretty clever idea! So, you want to buy 100k bitcoins right off, no time to wait? Buy 10. Put in deposit. Next buy 20. Put in deposit. Next buy 30. Put in deposit. Next buy 60. Put in deposit. Next buy 120. Deposit. 240. Deposit. 480. Deposit. You can now buy up to 960, and on and on. All this with no real risk for coinbase, all they expose at any one time is 10 BTC.
Dwolla
The BTC community doesn't like and doesn't use dwolla because on top of appalling service, hassling Bitcoiners etc it also either stole a lot of money from at the time 2nd largest exchange (Tradehill) or else helped them do it. Dwolla, much like Linode, are the antichrist.
If you're in a SEPA zone maybe try out bitcoin-central.net, they (unlike MtGox and everyone else) have the actual licenses to handle user funds, which are insured as with any bank etc.