I have my own theory for Sam forgetting about Donna - not that it would be a very popular one.
Aside from what we see in the episode "The Leap Back", we dont know what kind of marriage Sam and Donna had. It could be that Sam hasn't finished regaining his memory by the time he leaps out again. It could be that it's not really a peaches and cream marriage.
For all we know, the marriage could have been horrible and the real reason that Sam got into the accelerator chamber (in the changed history where he is married to Donna) isn't to prove his theories or risk losing funding but maybe the real, never revealed reason is that he was running away from a looney tune wife and a bad marriage.
I find it interesting that Sam remembers so much of his past and his personal life over the run of the series but never, except in "The Leap Back" and "Star Crossed", does he remember Donna. In "Dreams" he even refers to the simo-leap so it's not as if the whole event is removed from his memory - just the portion that includes Donna.
Over the course of the series he remembers his mother, his father, sister, brother and brother-in-law. When he sees Nicole in "Catch A Falling Star" he remembers who she is and how he had a case of puppy love. In "How the Tess Was Won" he remembers having a schoolboy crush on a teacher and being upset when she married the principal. Through all that the only time he remembers Donna is in "Star Crossed" and again in "The Leap Back". Maybe Sam's lack of memory of Donna has nothing to do with GFTW preventing him from remembering. It could be that Sam's not remembering Donna is due to selective amnesia and that he's choosing not to remember her. If, as "Mirror Image" would have us believe, Sam's in control of his leaping, maybe the reason he doesn't go home is that he doesn't want to face a failed marriage.
I also feel that Donna's inclusion casts Al into a bad light. Although Al might not be above cheating on the woman he's currently with, he knows that's not something that Sam would do. I think it's highly doubtful that Al would encourage Sam to do something that goes to far against what he believes is right or wrong. Yet, if Al knows of Donna and the marriage yet encourages Sam to pursue women (which he does) then he's doing just that.
I find Donna to be one of the biggest mistakes of the series. I'm really not sure what her inclusion was meant to accomplish in "The Leap Back" but I think all it serves to accomplish is to confuse things and to cast the actions of Al and Sam in a bad light. Sam has suddenly become a serial adulterer (because whether he remembers his marriage or not, he is committing the act of adultery everytime he has a relation with another woman) and Al has encouraged him to do so. Personally, I'd rather get stuck watching a whole season of Abigails than to have to watch Donna in even on episode.
Keep in mind, this is my opinion and it's certainly not anything written as hard fact. I guess it should also be noted that I truly do dislike and despise the character of Donna and consider her a complete waste of time. I'd have much rather it was Tom that was waiting in Control in "The Leap Back" and seen how Sam suddenly dealt with the brother who had been dead suddenly being alive. I can barely deal with Donna in "Star Crossed" but in "The Leap Back", I really can't stand her.
Putting in a shameless plug for my fiction, I've even gone so far as to write a story that creates what I think is a satisfying end to Donna (What Never Was -
http://www.fanfiction.net/s/3436757/1/What_Never_Was)
Oh, and I think the thread you may have been referring to, Isz, can be found here:
http://quantumleap-alsplace.com/forum/showthread.php?t=3678