Does Linux have a larger desktop market share than MacOS X? Probably not in the Small Business sector. You can get MYOB for MacOS X.
The problem Linux has in general is that because so much good software is free, very rarely does anyone need to buy anything. However accounting software is a good example of where the OSS community isn't very well served by commercial or free software. The only real decent options for Open Source Accounting packages are SQL Ledger (much better now than it every has been but still pretty rough UI wise) and GnuCash. GnuCash is fine for personal finance but seriously lacking in small business accounting features.
There are some commercial Linux accounting packages. The one I liked best was Quasar by the guys at LinuxCanada.com. It's a client server system and has really good POS options. It is lacking in some areas when compared to good Windows SB Accounting apps but overall is still good. And there is the AppGen system. But it is very old school and not very nice to use. There is also an advanced ERP system written in Java that I momentarily forget the name of.... And one I just remembered, Fitrix, which I've never looked at.
I personally use an old version of Quickbooks Pro with CrossOver office (WINE). It works perfectly. MYOB might work too. Accounting software must be a good market to be in. I know I dread the day that I have to convert from QB to something else. There is just so much data to convert and you would have to type it all in by hand as no Accounting Package can import data from another that I have seen. The downside is also that one solution does not fit all. Each country has different tax laws etc. Even from industry to industry there are many differences and things to consider. Hence why there is a good little local industry creating NZ and specific industry oriented accounting packages.
As fare as I'm aware there are no plans to make the product work on a linux based system, Unfortunately there is just not a big enough market to justify a linux release.