I agree with everyone that a 11-23 cassette is very tight! If you should buy a Shimano compact crank and have Shimano gearing now, then you should not have to buy another front derailleur. However, to answer your question about swapping wheels between the trainer and riding outside... if you have more wear on one cassette than the other, then you could definitely run into some shifting problems. The best thing to do would be to have a chain for each bike... you can put a SRAM chain on and you will be able to install it very quickly with out tools(once the original length is determined)simply by using the quick link that they use.

Good luck!