The number is the total of external and internal exposures, as of the end of March. The reason why TEPCO is now announcing? Well TEPCO didn't know, because they couldn't use the whole-body counters that measure the internal radiation at Fukushima I Nuke Power Plant.
Why couldn't the whole-body counters be used? (There are 4 of them at Fukushima I.)
Another Mainichi article (in Japanese, 4/30/2011) explains that there was no power at the plant until the end of March so the counters couldn't be used. By the time the power was finally restored, the air radiation level at the plant had gotton so high that the measurement was rendered irrelevant; even when the radiation was detected by the whole-body counter, they couldn't distinguish between the internal radiation exposure level and the environmental radiation level. TEPCO finally moved the workers who exceeded 100 milli-sieverts to its Iwaki-City facility and measured the internal radiation there, with the help of Japan Atomic Energy Agency.
If TEPCO was so disorganized and rattled with the on-going crisis at Fukushima I and wasn't paying enough attention for the radiation safety for the workers, wasn't it the government's responsibility to ensure the safety of the workers by arranging for the whole-body counters and doing the testing, much, much sooner?
(Oh I forgot. This is the government who said it was basically TEPCO's problem to find enough food, water and blanket for the workers, while it stood by, saying it regretted the situation.)
So it suddenly occurred to the government and TEPCO after 6 weeks that they could take the workers off-site and have them tested?
Mainichi Shinbun reports the news, but no other major newspapers like Yomiuri or Asahi do. Or maybe they do but I can't readily find the news as they are busy with the British royal wedding.
From Mainichi Shinbun (4/30/2011; my translation, emphasis is mine):
東京電力は３０日、福島第１原発事故の復旧作業に当たっていた作業員２人が国の規制の限度に迫る２００ミリシーベルトを超える被ばくをしたと公表 した。３月末時点の外部被ばくと内部被ばくを合計した。東電は「福島第１原発の計測機器（ホールボディーカウンター）が使えなかった」として内部被ばくの 測定が遅れたという。２００ミリシーベルトを超える作業員の確認は初めて。
On April 30, TEPCO disclosed that two workers at Fukushima I Nuclear Power Plant have suffered the radiation exposure exceeding 200 milli-sieverts, approaching the [emergency] limit set by the national government [250 milli-sieverts/year]. The number is the total of both external and internal radiation exposure as of the end of March. According to TEPCO, "The whole-body counters at Fukushima I Nuke Plant could not be used", and they couldn't measure the internal radiation exposure. It is the first time that any worker has been found to have been exposed above 200 milli-sieverts.
３月末までに１００ミリシーベルトを超える外部被ばくをした２１人について、優先的に内部被ばくを測定した。２００ミリシーベルトを超えた作業員 は、３月２４日に３号機のタービン建屋で電源復旧作業中に被ばくし、病院に搬送された３人の協力会社社員のうちの２人。最も被ばく線量が高かった作業員 は、外部被ばく２０１．８ミリシーベルト、内部被ばく３９ミリシーベルトで、計２４０．８ミリシーベルトだった。現在、残る１人の作業員と共に同原発での 作業はしていない。
TEPCO measured the internal radiation exposure of the 21 workers whose external radiation exposure exceeded 100 milli-sieverts by the end of March. The two workers whose total radiation exposure exceeded 200 milli-sieverts are two of the three workers from TEPCO's affiliate companies who were irradiated in the Reactor 3 turbine building [in the highly radioactive water] on March 24 as they were performing the electrical work to restore the power and were sent to hospital. One of them suffered 201.8 milli-sieverts external exposure, and 39 milli-sierverts internal exposure, the total 240.8 milli-sieverts. The two workers no longer work at Fukushima I Nuke Plant.
Of 21 workers [who exceeded 100 milli-sieverts], 8 had 150 to 200 milli-sieverts, 11 had 100 to 150 milli-sieverts. [And 2 exceeded 200 milli-sieverts.]
作業員（放射線業務従事者）の被ばく線量は、原子炉等規制法に基づく告示などで、５年間で１００ミリシーベルト、１年間では５０ミリシーベルトと 規定。緊急時には別途１００ミリシーベルトを上限に被ばくが許容されるが、国は特例で福島第１原発の復旧に限り、２５０ミリシーベルトに引き上げている。 【奥山智己、八田浩輔】
By law that governs the nuclear materials, nuclear fuels and nuclear reactors, the radiation exposure level of the radiation workers (workers who work in the radiation environment) is set at 100 milli-sieverts cumulative over 5 years, with 50 milli-sieverts as maximum per year. In an emergency, the exposure level can be raised to 100 milli-sieverts, but the national government has raised this emergency limit to 250 milli-sieverts for the work related to Fukushima I Nuclear Power Plant.