"Would regulators let the two biggest webmail providers combine?"

YES, they would.

WHY? because neither MS nor Yahoo! make any real money off of webmail (sans ads, a tiny fraction of their expenditures on the services)

All free email from either company amounts to commercially is a HUGE CASH DRAIN and either/both would probably be GLAD to dump it, if the Regulators got some bad Acid and objected.

AND, there are a GOOGLE-PLEX (get it?) of other companies more than willing to step in and provide free email.

NO FINANCIAL IMPACT TO CONSUMERS, NO REGULATORS INTERFERENCE...

"Let two major IM networks combine?"

SEE ABOVE, but MORE SO....there are SO MANY COMPETING IM services around available immediately.

and IM has NO DIRECT FINANCIAL IMPACT ON THE CONSUMER MARKETPLACE (as "IM GIANT" AOL's DYING Balance Sheet will be glad to testify)

"Allow MS to buy out a promising competitor to Exchange?"

By promising competitor, you mean what 2,3 or 4% of the marketplace?

other than off-platform PC-based competitors like *NIX, the only competitor on the Windows platform to Exchange Server is.....the Next Generation of Exchange Server

Take a look at Blue's acquisition of Lotus Development, and its impact both on Domino and Notes, that should resolve that question nicely.

"Allow two major portal sites to combine?"

When did, by marketshare standards, NOT financial nor advertising outlay or marketing expenses, MS' search efforts become "major"? And was there some "Internet Antitrust Act", that make FREE services subject to government antitrust jurisdiction, i missed?

BTW, if not for default OS installations and PUSH upgrading, the current marketshare of MS' Search efforts would be next-to-invisible, instead of hardly visible and largely irrelevant. Which is WHY they want Y!

ALSO, in many peoples' opinions, it would be huge mistake for MS to acquire Y! and then submerge the partially damaged Yahoo! search INTO MS' DOA search system.

IF MS is half as smart as they tell everyone they are, they would operate Yahoo! at a distance and keep pouring $$$$$ and talent into Yahoo! brand. And try to bring Y! back from the brink where it is right now.

If they just think that they can combine the two companies search efforts, rebrand it as MS-Yahoo! or Yahoo!-MS, and then VOILA', they will have a Google-Killer...

WELLLL, you can combine a duck and a turkey, but you won't get a pony, let alone a racehorse...