Jason is right the BR350 can not communicate on the same radio network as the BR340 that can communicate on the same ethernet LAN however.
It is physically possible to get this working in *some* situations with the right settings but the number of varriables involved as to if this will or will not work are too many. Links that have been *made* to work have not been reliable so I do not recommend it for a production network.
If you do play around and get this working you will leave you customer in a bad posistion for support and if the encounter a problem and ring the TAC for support then the their only option will be to use either 2 BR340's or 2 BR350's
The main reason for this incompatabilty was related to the way spanning tree protocol was implemented in the 340 bridges how ever turning of STP is not guarenteed to fix the problem.
The best way forward is to replace the existing BR340 with a BR350 and use the BR340 on another link that has a BR340.
It is unfortuant that they are not backwards compatible and as you may know Cisco works hard to make sure products are backwards compatiable but in this case, to keep it backwards compatible would have removed the ability to support some of the newer features that are now available on the BR350 we could not do both so the decision was made and this is the reason for the note in the release notes.
I am sorry I dont have better news for you.