Wrong comparison IMO...
Lets begin with "what is a Linux cloud" ?
The allmighty Google will point you towards "CloudLinux OS" (cloudlinux.com) amongst other things and that's also what this is about (IMO anyway). A "Linux cloud" is more a virtual Linux environment (virtual server) where you can do just about anything which you can do on a regular installed Linux environment. The only limitations you have is the power of the underlying hardware and the way the Linux environment was setup (for example; some providers don't give you any swap space and apply a hard memory limit).
Azure otoh. is basically a "cloud platform" which can provide specific services to build on. Databases, storage, etc.
So I don't think you can fully compare the two like this. Database on Azure? You basically decide the size using their "pricing calculator" where 1Gb already ticks to $9,99. You can pick the size, storage and bandwith (each generating higher monthly costs) but are still limited to 1 specific SQL engine.
Linux otoh... Well, its obvious enough: MySQL vs. PostgreSQL for example. Or both at the same time. The beauty here is obviously that your monthly costs won't be affected by all this. You rent a "Linux cloud" and what you do with it is up to you.
So Azure taking on Linux? Seeing is believing, and I don't think we're going to see it happening anytime soon.
Real life example: Right now I rent a few virtual Linux servers (CentOS 5). Approx. 60Gb storage, a good solid uplink, 1Tb bandwith, PTR records and the full yum repository at my disposal. Approx. $35,- / month per server.
Azure.. 250Gb storage (20Gb is hardly comparable to the above example), 2Gb database, also 1Tb bandwith and... Amazing, "only" $229,98 / month.
Do the math yourself here: http://www.windowsazure.com/en-us/pricing/calculator/advanced/
SO excuse me for not being a believer here. I'm not saying that there's no market for Azure at all, don't get me wrong, but no way that its going to come close to the extensiveness that Linux virtual computing provides.