Ask YC: MySQL vs. SimpleDB for startups?

Basically, currently we contemplating between MySQL db and cloud databases. We are looking for a scalable and reliable database. Consider large user data in multiples over 100k to 1M users. We understand that we need to make this decision sooner than later as it may get difficult to turn back. The things that worrying us are, 1. Is simpledb (by Amazon) is reliable? 2. Can professionals or enterprises or startups opt for simpledb. 3. what roadblocks we may get considering long term plans, if we opt for simpledb.

Please feel free to point databases other than these.

Even citation of other articles is appreciated.

Thank you.