You can buy Android powered smartphone's for less than £40 now brand new.
It probably isn't the cost of the phone that is excessively prohibitive, but the cost of the Internet/web mobile data service on top of voice/text. Fortunately, these smartphones are functional with just a Wi-Fi connection and no mobile data service is needed. (I'm presuming a person can use a smartphone but only subscribe to standard voice/text, or maybe not, I'm not sure.) If merchant and local exchange agents can provide Wi-Fi (at the least for accessing the blockchain or for web-based Bitcoin EWallets) then not every Bitcoin user with one of these smartphones ends up having to pay for Internet/web data subscriptions. But still a £40 device used just for use as a digital currency wallet is not going to foster wide adoption.
I've read there are bitcoin SMS wallets being developed?
There is Coinapult's SMS wallet, but that only works for U.S. and Canadian mobile numbers. WalletBit just launched an SMS mobile wallet that eventually supposedly can send to unregistered mobiles but today it required an Internet/web data connection to be able to follow the Claim URL.
A way is needed tho to make bitcoins to seem more attractive to them then Mpesa already is.
Well, technically an SMS EWallet system that works like hoow M-PESA works is easy to build. (Coinapult built theirs in a matter of weeks, if I remember correctly.) The problem is that text messaging is not secure. There is no ability to know if the text came from the handset or if it was spoofed. M-PESA doesn't have this problem because merchants are supposed to get info from a sender's gov't ID when a purchase is made and when redeeming or exhanging M-PESA e-float for fiat (i.e., exchanging out).
So this system would work until scammers figure out that fusnd from a person's account can be spent by simply spoofing a text message from a mobile number. That would probably have a half-life measured in days.
Thus it could very well be that there is no method where bitcoin would work with feature phones by themselves. But smartphones change all that as you can do client-side encryption on the phone before transmission (over SMS/text messaging even) or simply use a secure channel (e.g., https) on an Internet/web data connection (subscription or Wi-Fi). [Edit: This is assuming the mobile itself is a secure operating system, which is something that I'm not comfortable believing. If you buy an $80 smartphone from a carrier, is it a managed device where the carrier can put software on the device? If it isn't open source, you can't prove they aren't able to do that.]