The Mobility HD 5470 is actually a pretty weak video card. It would be slightly weaker than the desktop HD 5450 which is the slowest desktop Radeon 5xxx card. You can generally play current games at the lowest settings with decent performance. BF3 is going to be a DX11 title, and DX11 effects will lower performance a bit. As for Skyrim... it is unknown how well it will run since Bethseda is developing a total new engine for that game; Creation Engine. Since it is going to be released for consoles as well, there will definitely be a DX9 mode so a Mobility HD 5470 should be able to run the game with low graphics settings. You are not going to get 60+ FPS in SC2.
A slightly better option would be a laptop with an i3-2310m CPU. It comes integrated with the Intel HD 3000 graphics core which can actually outperform the desktop version of the HD 5450 by a little bit. Additionally, the newer Sandy Bridge Core i3 is about 10% more powerful than the older generation Core i3 clock for clock.
An option would be to wait for new laptops based on AMD's upcoming APU (Accelerated Processing Unit); Llano. Laptops based on Llano will probably be released within 1 month; best guess. Like the Sandy Bridge i3 CPU, Llano has an integrated video core. There are a a few graphics options for Llano that revolves around the A6 and the A8 graphics core.
Overall, the graphics core in Llano should be better than the Intel HD 3000 graphics core. However, CPU processing power will be less than that of the Core i3. Generally speaking, most games are limited by the GPU as long as the CPU is fast enough.
Here's a performance chart that include two Llano A8-3500M APUs; one has the 6620M graphics core, the other has the 6630M graphics core. Note that using FRAPs to measure frame rate imposes a slight performance penalty; maybe about 5%.
The A6 series of comes with the 6520G graphics core which is rated at 72% of the performance of the 6620M graphics core. Or basically almost 30% less performance. You can read up on the Llano APU in the following thread.
Regarding TF2, you should have good performance. It uses the Source Engine which does not have high requirements. You can look at the benchmarks for Left 4 Dead 2 which also uses the Source Engine to get a sense of performance in TF2.
For comparison look at the 13" Acer AS3820TG Timeline X Notebook It's over $400 less expensive and lighter @ 1.81 kg / 3.9lbs.
Core i5-480M 2.66Ghz CPU, 4GB RAM, 500GB HDD, 13.3" 1366x768 LCD w/ HD 6550 graphics.
It doesn't have a DVD burner installed but you could buy an external model and not have to carry extra weight when one isn't needed (which is most of the time).
Acer Aspire 3820TG Review (an earlier model with i3 CPU and HD 5470 graphics).
The switchable graphics should give you a nice battery run time advantage over the ASUS B43 and the HD 6550 is a nice boost in gaming graphics power. AMD Radeon HD 6550M review
And the newest Sandy Bridge model is an even better gaming option: Acer Aspire TimelineX AS3830TG-6494 $700
Core i5-2410M (2.3GHz), 6GB RAM, 750GB HDD, 13.3" 1366x768 w/ NVIDIA GeForce GT 540M 1GB
Also has Bluetooth 3.0, and USB 3.0.