It seems that the eruption at Lengai is indeed a major event and that explosive activity has taken or is taking place in addition to lava flows and probably, bush fires. According to local sources from the nearby Moivaro - Lake Natron Camp, Lengai first erupted at midnight and at 6 am on 4 September. A pilot reported ash rising to 20,000 ft (ca. 6 km). Ashfall was reported from Engaresero Village, but there are no reports on injuries or fatalities. The highly regarded Moivaro Tented Lodge near Lake Natron has been temporarily evacuated.

As more news has become available, the news about an effusive flank eruption at Lengai around 15 July turns out to be probably false. The observed "ashcloud" was dust from a landslide. This demonstrates how difficult it is in this area to have robust information. The ongoing seismic activity in the Gregory Rift, however, is quite peculiar.

Thomas, our correspondant in Arusha climbed Lengai last week and reports indirect signs of activity near the western rim of the crater, at the place where lava overflowed in the March 2006 eruption.
this is his report:

Glow from nearby (but hidden) lava pooling somewhere beneath the collapse pit in Lengai's crater (taken on evening of 31 Jan 07).

During our latest expedition to the active crater of Lengai volcano from early 31 Jan - 2 February 2007, no eruption of natrocarbonatite lava occured from the summit vents. According to local Maassai guides, the central area of the crater with the large collapse pit near the tall hornito T49b had not seen significant changes since last summer. 
From an open vent in the NE corner at the bottom of the pit at the base of the hornito, loud washing-machine like sounds of boiling lava was audible all the time. A large pool of lava must be found somewhere at very shallow depths in some caverns beneath that area. This assumption is confirmed by the glow of lava that was clearly visible one night from a second, smaller vent located about 30 m south of the large vent in the base of the collapse pit. One guide confirmed he had seen spattering of lava from this vent some two weeks earlier.

More detailed information about the recent activity at Lengai has emerged: there was probably no explosive eruption as reported earlier, but one or two very large effusive paroxysms: the first eruption occurred on 29 March, the second probably on 3 April. The eruptions produced large-volume natrocarbonatite lava flows that overflowed the crater to the West near the climbing route, travelled for more than 1 km length through a small canyon before stopping less than two km from a Maassai village at the base of the mountain.
Buring vegetation ignited by the lava might have produced smoke that had been mistaken for ash. No evacuations have taken place and no one has left the area, different from early news reports.

The activity at Lengai volcano, that started around 30 March is still enigmatic. According to information compiled by our colleague Fred Belton, a lava flow and lava fountains were emitted on the 30th, but there were no ash emissions. A steam plume was visible that may have been mistakenly described as ash in news reports. According to an observer in Tanzania, on 4 April a very large lava flow was visible on the volcano's W flank. The lava flow was over 1 km long and had traveled down the flank of the volcano and into a gorge. There were no signs that the flow was still hot. Photographs revealed that another lava flow may have traveled W of the volcano on 3 or 4 April. On 7or 8 April, active lava was contained within a new lava lake (?) at the summit. There are contradictory reports about evacuations evacuations from villages near the volcano.

There has been a possibly major summit eruption at Lengai volcano on 31st of March 2006, involving significant ash fall and lava flows from the summit, according to the following news report, posted on IPPMedia: