O2 will be offering 4 new phones in April: the HTC Desire S, Sony Experia Arc and the Sony Experia Play, which is notable for it's Playstation-centric store/interface plus a slding gamepad featuring analog and digital joysticks. Could be quite popular. There is also a tablet on its way.
T-Mobile have announced the imminent arrival of the LG Optimus Black to their network. A nice looking and very slim Android phone but single core only.
Vodafone has the largest selection of coming soon Android phones at time of writing this article:-
Samsung Galaxy S II - sequel to the highest selling Android phone to date - the Samsung Galaxy S.
HTC Desire S - arguably the most popular model from HTC gets a refresh. HTC has the most disappointing 2011 smartphone lineup of all the big players, due to their contract with Qualcomm. Qualcomm is taking it's time bringing its own dual core mobile CPU to market, so no HTC phones announced at Mobile World Congress feature a dual CPU. However, Qualcomm's MSM8660 has two Scorpion cores clocked at 1.2GHz and the Adreno 220 GPU so they could end up having the last laugh.
Samsung Galaxy Tab 10.1 - the honeycomb tablet from Samsung.
The Sony Experia Play , Arc and Neo -which the full complement of new Sony Android phones.
Samsung Nexus S (available now from Carphone Warehouse. )
Three is, at time of writing, the only carrier offering the LG Optimus 3d, which recieved some stunning benchmark results over at Anandtech. Set to arrive in June.
The OMAP4 / SGX 540 seems to be the dual CPU/GPU combination to beat right now.
Three is also offering Experia Arc and Play, along with HTC Desire S in April. The Samsung Galaxy SII in May.
UPDATED: 4th March 2010 - Nothing of note has appeared on Orange's website as yet, so I can't report anything in regards to what they will have on offer later in the year. Suffice to say that as of yet, the Motorola Atrix 4G has not been announced for any UK network yet.