Sri Lankan (UL) join date?
 

Does anyone know an approximate date when UL is scheduled to join OW?

It usually takes 12-18 months after the joining announce. UL was announced in June so at least June 2013, but it's more likely to join late 2013 or early 2014. They have to integrate their systems and receive the approval of the sponsor airline (CX in this case). Consider also QR's full integration may have a priority in the plans of oneworld.

For comparison, Malaysian was announced on 6 June 2011...
It is still not a full member - scheduled for 1 Feb 2013 (20 months),
albeit earning rates are yet to be published

Qatar may enter before Sri Lankan cause they have nearly done everything

Do you have sources to say that?
I'm not polemical, just asking for traveling reasons :)

What have they done?

To be fair, it was reported that MH was ready back in Nov 2012 but delayed their entry until the peak holiday season ends thus Feb 1. So 18 months exactly.

Who reported that?

Per the 3rd paragraph of the the oneworld web site:
http://www.oneworld.com/news-informa...objectID=52929

Although given Sri Lankan was announced in June 2012, it runs into the same Xmas period at 18 months
1 Feb 2014 could well be its potential date

I suspect that QR's join date will be mid-2014 !

Why is it that new potential Oneworld members take ages to integrate into the system whereas *A and ST new members seem to come online very quickly?

I wouldn't only say it takes one worlds new members ages to come online it seems it takes most members around the same time. Star Alliance had Air India joining for 3 years before revoking their membership. Garuda is taking 3 years to join sky team so its not just oneworld and 12-18 month joining process seems to be normal.

Sri Lankan may join not so later due to its "size": it's a small carrier of a small nation, integration could be faster than others (see MH).
I guess Qatar wants to be fully integrated at the time new Doha airport opens: my two cents are on that date.

I thought that's why OW moved to the cloud network system. It is supposed to expediate new member integration from now on. In the past each OW members have to set up individuals "links" (direct access). Now they just have to link to the OW server.

They also quoted 'administrative reasons', which suggests they weren't completely ready, it is an administrative process after all. Alternatively, perhaps they were genuinely surprised that the northern European Winter/Christmas holidays was going to be a busy time..?

MH used to aspire to SQ and CX, but they are quite third tier, falling below TG service standards.